From a technical standpoint the differences cannot be huge. Custom GPTs can only add so much. But where Gurobot exists mostly to answer all manner of API and modeling questions, Gurobi AI Modeling Assistant was created specifically as a support to the linked documentation and is focused on modeling.

There is now gurobi-ai-modeling.readthedocs.io which is aimed at the non-mathematician crowd for building optimization models from human-language problem descriptions.