I don't think there's any straightforward way. You don't say what version of the game you are using (Definitive Edition?) but as far as I know Gaia units have worked that way in every version.
But it sounds like you're willing to use some triggers, and I think it can be done.
Have the units and buildings start not as Gaia it as belonging to some AI. You can change ownership of them all to Gaia immediately after the game starts. The behaviour you're trying to avoid comes with things that are placed as Gaia to begin with.
Then use the Objects in Area condition to detect when a human player or AI is near, and the Change Ownership effect. Because you want each unit or building to be captured only once, make sure that each trigger deactivates the equivalent triggers changing ownership to other players.