GSX surely works perfectly fine in MSFS 2020 after installing MSFS 2024. If you have both installed, you saw a message telling you that Airport Service and Jetway replacement files, which are shared between the two, will be disabled in both, explaining this is entirely normal.
Disabled Airport Service don't have any effect on GSX operation, they only control the *accuracy* of default ground vehicles operators, which will be assigned "by region" instead of "by airport", but those are just the default vehicles servicing AI or wandering around the airport. GSX vehicles servicing your airplane won't be affected in any case.