The answer from the developer is missing here:
To clear up the above before anyone gets confused. There are lots of different kinds of "open source" and different ideas about what "open source" should mean and they are not all compatible with one another.
For every "open source" project there is a license that dictates what is and is not okay to do with the product, if you care about such things you should read the license and not just assume that open source means "anyone can copy".
Gulden is not "open source" if you go by the strict OSI definition
https://opensource.org/osdGulden is open source for the definition of open source we consider important:
* Anyone can view/inspect our source code
* Anyone can compile our code
* Anyone can modify our code for personal use so long as they don't give it to anyone and/or as long as it is not modified to work on a different blockchain.
* Anyone can submit patches to our code etc.
This explicitly does not include copying/modifying the software for the purposes of using it on a blockchain that is not the Gulden one, or distributing it to other people etc. without the express permission of the Gulden developers.
So to answer the original question:
1) They would be in legal violation of the licensing.
2) The amount of work being done here is far beyond the scope of something a "copy and paste" developer could copy/paste into another existing project successfully. The average copy/paste project differs from bitcoin by only a few hundred lines of code, there are dozens of documents explaining how to make such a project, and even then the 'copy/paste developers' of these projects tend to struggle and make obvious mistakes, Gulden differs by around 40k lines of code at this point and growing, much of this code is of high complexity and easy to mess up, there are no instructions on how to copy this code...
3) This is a lot of work, it is much easier for such projects to just make up fake features like they already do - so I imagine they would prefer to do that.
4) Even ignoring the above we would have first mover advantage and all that they would achieve is to draw attention to us as the original developers, meanwhile we will already be developing the next thing.
In short; I don't really think it is a legitimate concern, but even if it were, so what? Should we just throw our arms up in the air and give up? The way I see it there is only one thing to do, keep going forward...