1. Which license do you use ? (GNU or share alike ? or something else?)

Actually, I'm not really clear on why licenses are necessary. The only restriction I can see wanting to put on the work would be one to force anyone who builds off of it to also release their work free from all other restrictions. I suppose that's possible, at least in the sense that you can write a license to say that. I'm really not sure if it's the sort of thing anyone is ever going to enforce. Like, if Hewlett Packard releases a commercial 3D printer based on the RepRap, but doesn't make it open source, could Dr. Bowyer take them to court? And, if he did, what would he be awarded if he won? He'd get to spend all that money on legal fees, and the outcome he'd be looking for would just be for HP to publish their design and manufacturing files? That doesn't make any sense to me. 

2. Why do you choose the open source solution? What are the advantages of your project?

The first reason is that I don't need more money and I don't get excited about starting a business. The second reason is that open source projects are capable of moving A LOT faster than closed source projects. Because of that, they attract some of the best people in the world. Open source projects are really just about solving a problem in the best way possible. The profit-motive definitely provides incentives to solve problems, but the possible solutions are restricted by the need to turn a profit in a certain period of time. Well, open source projects don't have ANY restrictions on their solutions. Because of that, they produce better solutions, and they produce them faster because anyone in the world can understand and contribute to them at any time. That's a huge pool of creativity; far larger than even the biggest global corporation. 

Additionally, open source is self-reinforcing. When you give your solutions away it relieves other people of the need to keep their solutions to themselves; there's no competition, just cooperation. That means you get back several solutions from other people that you never would have thought of. You got those solutions for free and they'll pay dividends forever. Open source removes the need for money. Rather than fight each other over a zero-sum prize like profit, we can work together so everyone gets helped. 

3. How do you get your project known? (advertising)

I talk about it to anyone who will listen. I also blog. 

4. Do the GVCS give a guarantees ?

Not a safety or performance guarantee. At least, not for work performed by other people. Once the GVCS is completed, OSE is planning on selling training and certification to anyone who wants to start a business fabricating (and maintaining) OSE machines. So those people will be able to display the OSE Certified logo, which might be worth something in the future. 

But, in the grand scheme, the GVCS is the equivalent of those instructions you buy out of the back of science magazines. Like, "build your own recumbent bicycle." Because you're the one building it, the guy who made the plans can't give you any guarantees. The only guarantee you can rely on is that it's always illegal to encourage people to hurt themselves or others, so OSE isn't going to distribute plans for machines that are grossly negligent in their design. 

5. Do you produce the machines completely open from the beginning or do you first developed them to go later open? so the community can "only" improve them.

That's actually a really good question. Open source, as a coherent movement, started in the software world because it's really really easy to trade and experiment with code. Then it moved into the electronics world because, while actually building something is harder, at least electronics don't take up that much space, and there are even business who will build whatever crazy idea you have for a small price increase. Now open source is moving into the world of HARDware, so it is confronting the laws of physics. Brianna Kufa is designing and building the ironworker because she has full access to a working metal shop. But, even with that, she has issues of cost and mass. Stock metal costs real money to buy and ship then, once you get it, just moving it around the shop requires special equipment or several strong men. It takes longer because you literally can't afford to make a mistake. Building something out of it requires college-level math and specialist skills. So you can't even learn how to work with the stuff without money and time. 

So, while the ironworker is totally open source from the beginning, it effectively is closed source because 1) very few people in the world even know what an ironworker is and 2) participating in the build requires a significant investment of time, money and space. When the ironworker is finally finished, and the design has been validated, and all the plans and fabrication instructions are released, you're still going to be talking about a 6,000lb machine that is only useful if you need to work with an awful lot of stock metal. Very few people in the world are going to care about it. The ones who do care are not going to just "jump in" because even the cost savings of an open source design don't entirely offset the inherent expense of a machine that big. It's not just a matter of the machine itself, you have to pay to transport it, you need a special foundation to place it on, you need special tools to assemble it, and you're not going to want to move it again so you need to have already laid out the shop it's going to be a part of.

A lot of the machines OSE is working on are industrial/agricultural, so they have the same issues. Sure, they are open source from the beginning, but they aren't the sort of things that get anyone excited and build a following of active developers. So the fact that they are open source is moot.

6. Why does people join the project ? Are there any "developing competitions" with prices or something like that. Or do they join just because of beeing interested in working together at those machines ?

I don't think I've ever heard of a prize competition for open source designs. That would probably be missing the point. People obviously don't join open source projects for the money because there isn't any. The reason I join is that I can't stand to see anything working less well than it theoretically could work. Open source projects are the only place where I can get onto the team and help improve the design. Additionally, I like the idea of people around the world gradually creating an entire network of open source options. Because they'll be better than their closed source counterparts, and because they'll  adapt to changing requirements faster, they will gradually replace the things that don't work as well. That will be a paradigm shift in how the human race operates. It's fun to think about being a small part of that revolution.