We opened our tank after our pump started cycling far too much during heavy rain. Turns out we have a groundwater leak in the seal where the inlet pipe comes into the tank. Something is wrong with the gasket seal we assume. Question is, can this gasket be replaced with taking out the pipe and replacing the whole situation? If not, can we use some sort of sealant that will adhere to both the abs material of the tank and the rubber of the gasket, while also withstanding the pressure of the groundwater from behind the tank wall? Hoping for miracles here! Thank you!!!!

the rubber gasket is replaceable.
problem is there are many styles.

So does that mean I can’t use a different type of new gasket to replace the old one?

it is not how the gasket seals the pipe that matters.
it is how the gasket seals the opening in the tank that varys by manufacturer.
remember the tank is round so the opening will not accept a gasket meant to seal a flat circular hole.
just as an example take a look at this design.

Ah, I see what you mean now. Ours is just a rubber gasket so flexible. We ended up just using silicone around all sides of the gasket in two coats. Fingers crossed! We also notice that there is a slow drip coming from the inlet pipe itself even though there’s no water being used downstairs. Where could this be coming from? Does this mean there is some sort of water leak somewhere?

If the bathroom has a vent through the roof it could be rainwater or you have a running toilet/ faucet or maybe condensation

yeah, generally means the flapper on a toilet is not sealing 100%