Zeddy uses a public Boss HUD plugin, probably that one from AntiTeal.
The things I say below are pure speculation from research I got over few months so please correct me if I'm wrong:
GFL uses a private boss HUD plugin (from PSE maybe) which is called Boss Rank or B-Rank (?) that is far more extensive than AntiTeal's.
Features for example are a boss HUD with a custom boss name which you can set in the .cfg (more about it below), a ranking system of who inflicted the most damage to the boss and a hitmarker which is really nice.
AntiTeal's Boss HUD is fine, but if you enable it, it will detect every single func_breakable/math_counter/func_physbox. So not only the boss will get a HUD but also i.e breakable doors etc. GFL's plugin on the other hand works like entWatch. You can actually pick each entity (func_breakable etc.) individually that might belong to a boss and write it down into a .cfg and only those entities will be detected by the boss HUD plugin. That means the actual bosses in the map will get a hitmarker, HUD, damage ranking unlike other unnecessary entities (door, wooden wall, boulder etc.) like above mentioned.
So in fact, we do have Boss HUD on GFL.
The problem now, some .cfg might be outdated like for example ze_dreamin.
ze_dreamin_v2_1, the one before the current version definitely had Boss Rank (hitmarker etc.) and after it got updated to v2_3 it stopped working.
So I assume the .cfg has never been updated for v2_3. That is why some maps have B-Rank and some don't.
I could list way more maps that had Boss Rank working before and stopped working after those got updated.
But back to the topic. I agree on Mercy's suggestion to enable Boss HP etc. globally for all players and not for admins only, but then it would require to update maps with B-Rank configs which neither me nor everyone else outside the management (admins etc.) know how to make.