Well gpcosim is not useful without iverilog, is it?

Theoretically you should be able to use another Verilog simulator along with gpcosim (e.g. a proprietary one). Not sure why exactly you would do that, but...

iverilog's vpi_user.h's license (GPLv2+) is not compatible with openfpga's (LGPLv2.1+).

@azonenberg and I discussed this IRL at DefCon and he mentioned that he isn't a huge fan of VPI and might consider a completely different interface for the cosimulation code. He mentioned that he has/will have something that just uses $fread and $fwrite on pipes as an example of one possible way to build a new cosimulation framework. The IRL discussion then spiraled into a discussion about whether the potential new framework would be used for cosimulation, mixed-language simulation, or both, and then the discussion turned into a bikeshed about what exactly needs to be expressible in a potential new framework. No conclusions were reached, but I'm just writing this down so that we can remember that this happened.