Does a hero in the Great citadel of Ares provides the GCA special effect?
Just for reference the description of the citadel:
As long as 1 or more of your troops are stationed on this territory, it cannot be invaded by sea, land (Titans) or air (Pegasus). However, this territory remains affected by other effects (Harpy, etc.)
Honestly, I don't know for sure. I've looked up the Hades rules for a description of heroes and they state something like "Heroes count as equivalent for troops in a battle." So, they are not exactly troops, but I would probably treat them as such. Cyclades rules are not written that water proof with all the cross references between different expansions. So, sometimes you have to guess the spirit.
I would allow the effect, as it also makes sense thematically, but Matagot/Bruno should have the last word, of course.
See also this thread as I would prohibit all kind of invasions.