In terms of defence, fighters can already make use of ground-based maintenance facilities. They just can't reload as quickly. Maybe the solution is a new 'Airbase' installation, that allows reloading a certain tonnage of fighters on the surface at hangar speeds (which could only be used by fighters due to 500-ton restriction on planets), without the overhead of managing fighters in hangars
It's not merely about maintaining fighters around a planet, it's about being able to station them on the ground, utilizing the atmosphere for cover so even if the battle in space is lost, surviving fighters could still contribute to the battle on the ground without being taken apart by the enemy fleet. The key difference here is that any fighers thusly deployed can continue to rest and resupply on the ground without entering orbit to do so where they will be instantly destroyed. That's the only way I can think of to allow actual defensive use of GSFs and allow the player, who is far more often on the offense than on the defense, to fight against them.
I'm suggesting some sort of a secondary Maintenance Capability pool provided by the new Airbases for fighters only, that can be selected as a target for Fighter Factories or fighter-manufacturing shipyards, can't be exceeded (excess fighters overflow into space), treats them as parasites (full maintenance/resupply/etc, can't be acquired as a contact or targeted, every destroyed Airbase results in the loss of a percentage of total fighters) and can either launch them into space as a fleet or into the atmosphere for a ground support mission of the owner's choosing.
The possible case for possibly making this new Airbase a GU is that there's already a precedent of certain GUs (STOs) having a separate contact associated with them, and I'm fairly sure having one for Airbases would make sense as targeting enemy airfields is a staple of modern warfare, although this might also be entirely counterproductive unless they're exceedingly difficult to destroy from orbit, as that would nullify ground defense fighters as a threat in the face of enemy space supremacy.