About Jetways:
GSX doesn't have ANY control over how well (or how bad) jetways works, especially if the airport has GSX replacement jetways Disabled, something that should always be done, if the airport is 3rd party or is an Handcrafted/Premium airport. GSX replacement jetways can ONLY be used on bare basic default airports and nothing else.
Assuming your correctly configured the GSX replacement jetways to be Disabled on all airports they are supposed to be Disabled, if the jetway doesn't work, the problem is then completely unrelated to GSX, which in MSFS ONLY sends the standard Jetway Toggle event, and nothing more.
Whether the jetway works (or not), it's completely unrelated to GSX, and it's a combination of your parking position, the airplane doors, how the jetway is modeled and how the jetway is placed in the scenery.
Since one of the things that affect the jetway is your parking position, you can indirectly improve the chance a jetway would work better, by customizing the STOP position in GSX in an airport profile. The manual has a very precise explanation in the chapter named Understanding the Stop position Pages 48-49, no use repeating it here. Of course, this can't fix every possible case, only the ones where the only reason why the jetway didn't work, was how the airplane was parked.
About Parking too small:
GSX just read the parking radius has set in the scenery, and compares it with the airplane wingspan, to decide if the airplane can fit the parking spot. Of course, this is just the default behavior but, there are plenty of places in the manual where is explained how GSX decides if a parking is too small, and the multiple options you can use to deal with it. For example:
- The explanation which tells you can still use a "too small" parking, if you really want, at Page 13.
- The explanation of the Max Allowed Wingspan parking customization feature at Page 39.
- The explanation of the Ignore Wingspan when parking global setting, which is used to tell GSX to always ignore the parking size, at Page 32.