You are a bit wrong on the requirements.
For anything to be trainable/buildable it needs:
1) Proto: Train/Build line in the unit doing the training/building.
2) Techtree: Unforbid and Enable Unit. This mostly happens through Age Techs, like Age 2 Freya allows all Age 2 Norse buildings and units. Age 1 just allows the basic units for every civ, like Walls and building one TC.
3) IF "allowed culture" is stated, the unit will only be available to the allowed culture.
4) IF "allowed age" is stated, the unit will only unlock with the allowed age.
As I wrote, you can make Gastraphetes (in protox crossbowman) with any god, but the allowedage requirement needs you to be age 4 for that.
You will just need:
(1) A greek fort,
(2) unforbid and enable crossbowman,
(3) No culture restriction, so any god,
(4) Set Tech Age 4
For the Fort you will need:
(1) A greek villager
(2) Unforbid and enable Fortress (or Age 3 greek god)
(3) No culture restriction, so any god,
(4) Set Tech Age 3
For the Greek Villager you will need:
(1) TC
(2) Age 1 Greek or Unforbid and Enable Greek Vill
(3) Be greek.
(4) Just Age 1 is enough.
... so if you place greek vills for an age 4 player (maybe you need to enable age 3 too), he can have forts and gastraphetes if you want him to.
The game works perfectly fine on the first two requirements, but in some cases the 3rd and 4th are stated too which make scenario making tricky. Well, mainly the third one keeps us from Priests making Obelisks, Temples making Priests and Longhouses-Ulfsarks-Combinations building each other. But using the exceptions some things can be done. E.g. both atty barracks and the palace, probably also all atty units should be available for all players with atty cits and the right age. Even "Villager Atlantean" is not culture restricted, so you can forbid all other vills and allow atty vills. No matter which god the players can go full atty pretty much. Additionally I believe all Myth Units can be just enabled too. Centaurs from Norse Temples look somewhat fitting. You could make a RMS, forbid greek/norse/egyptian villagers and the starting units from their TCs, allow all atlantean units and buildings (except myth units). The age restriction on the buildings and units will work out just fine here. So an Odin could go Frey and build murmillo/turma in 2nd age together with valkyries. A Ra could go Sekhmet and have Scarabs and Destroyers. God Powers like Son of Osiris and Ragnarok wouldn't work though.
[This message has been edited by Izalith (edited 03-29-2018 @ 10:44 PM).]