I don’t understand the concern about licensing images of your project. GF isn’t demanding the source files as a condition to you sharing your project. They want pictures to use to show other users why those other users should buy a GF. They want permission to do whatever they want with those pictures, as a means of effectively leveraging the images for their marketing efforts.
Could someone reproduce your idea from the pictures? Could GF? Sure. Especially if you post orthogonal views that are easily traced. You can make it a lot harder by taking “artful” shots that introduce perspective that’d confound simple tracing. But even if you don’t, it won’t be your artwork, it’ll be their attempt to duplicate your idea. You’re no more exposed than if you posted to this forum, or started marketing your product. Anyone could see (or even buy) your product and then duplicate it. What’s the big deal with GF’s proposal? There’s no real downside, and the upside is that you might get people asking you to make your idea for them.