Decided to get hold of Asset Forge at half price to take a gamble on this. (Thanks for running that birthday discount, Kenney!) It turns out that although .glb doesn’t appear among the list of model export options within Asset Forge, choosing to export as GLTF actually produced a .glb model that works very nicely in GDevelop:
My initial test models are pretty basic, but getting them into GDevelop was absolutely trivial, which is exactly what I was hoping for. I’ll most likely upgrade to the deluxe version of Asset Forge to chip in a bit more money for this. I’m not 100% sure how much 3D stuff I’ll be doing but the software is fantastic to use so far.