More of my installed 3rd party add-on airports start showing overlapping parking spots. At gates with jetways this causes GSX to deactivate the jet-bridge and I'd need to use stairs instead. If I activate the jetway in settings I get informed the jetway don't work due to "Bad Jetway Data". at airports w/o these overlapping parking spots (even if they are in the "Enabled" column in GSX Config Menu) everything works fine!
I think you fundamentally misunderstood what the GSX Config page does and how GSX replacement jetways works. You said you are using 3rd party airports and, at 3rd party airports you MUST Disable GSX replacement jetways, always and no matter what.
The GSX replacement jetways are made exclusively for DEFAULT (bare default) airports, cannot be used for anything else, no handcrafted airports, no custom airports only available on premium version of MSFS and surely not 3rd party airports.
The Config page there, is not to "Enable jetways", is to PROTECT non-default airports from having their own jetways replaced by GSX! You are allowed to choose airports ONLY to add the extra ones (usually freeware) that, due to their non-standard naming used in their package folder, might have not automatically detected by the FSDT Installer to be excluded, that's the only choice you have but in general, ALL non-default airports must be on the Disabled side.
The duplicated parking spots on the map were caused exactly by not having done that, which is what the manual explains, at Page 7:
If you install a new 3rd party scenery, just installing it won’t result to automatically go here and reconfigure Jetway Exclusion, you should develop a habit to always check the GSX Config page after installing a new 3rd party scenery. Failing to do so, would result in Parking conflicts (visible as double Parking spots in the World
Map), and Jetways malfunctions
Then proceeded to shift the airport(s) with overlapping parking spots in GSX Config menu from "Enabled" to "Disabled" column and restarted MSFS2020. The overlapping parking spots were now gone and the jetway was working fine
Of course they work now, that's how you were always supposed to use the GSX Config page for 3rd party airports.
but when I requested boarding I was asked to open exit 2 (was already open) and if I require crew boarding (of course 
I need the crew!), However no crew appeared and also no pax (even GSX was saying boarding in progress). still, baggage was loaded normally ...
This is a completely different issue, which can't be answered without knowing which airplane you are using and how it's configured.
