Trademark law leaves no space for that. The Lego Group has to actively defend their trademark. That means a name like LegoGPT is really on the obvious end of 'don't do that'.
Completely agree. This should be well beyond accusations of corporate bullying. It's one thing to mention Legos, it's another to actively include a brand name in your product! NikeGPT, CocaColaGPT and IkeaGPT will face the same issue ;)
Mentioning Lego is absolutely OK, and you can sell used Lego as well and note that you are using genuine Lego bricks (resale laws simply allow that). Lego is really antsy about anything which might look like it is actually a Lego Group initiative though, and anything where Lego bricks are offered for sale in a modified state¹.
European trademarks are registration-compulsory, hence the required extent of defence is significantly weaker than those required for keeping the common law trademarks.
Also, they don’t tend to go after fan-made things like this, based on some googling they typically throw the book at counterfeit producers who are eating into their profits.
(Initially) fan-made stuff which gets big enough to get noticed usually won't be able to call themselves something with 'Lego' in it. Usually some variation of 'brick' is used instead (e.g., Bricklink, Rebrickable, EuroBricks, etc.).
Backing by a multimillion dollar University for publication and promotional purposes is a far cry from hobbyist and enthusiast ecosystems, at least in my view. Then again I’m in a very slim minority of actual creative creators who generate IP from scratch and my perspective is much different than “move fast break things” attitudes.
