The Maya case established that GC beliefs were protected because they met the Grainger test.https://en.m.wikipedia.org/wiki/Grainger_plc_v_Nicholson
The original Maya ET case found that GC beliefs met the first four elements of Grainger but not the fifth (worthy of respect in a democratic society/not interfere with the rights of others). The EAT found that GC beliefs met the fifth element too, so GC beliefs are protected beliefs.
GI beliefs have not yet been put through the test, because no-one has yet been sacked for their GI beliefs (despite trans people being the “most vulnerable” in the world). While it seems likely they would, if tested in court, meet the first, second, third and possibly even the fifth limb, the one they might struggle with is the fourth one, about attaining a level of cogency. That is not to say it would definitely not pass, just that that is where the heaviest lifting would be required.