I'm not sure what do you mean by "slant". If you mean the jetway will have different pitch when docked to different airplane types because of the different door's height, it's nothing that concerns you as a designed or GSX, because the first waypoints will be supplied by Simconnect, you only need to set the last one, which is the height at the root
Check the explanation at Page 77 of the manual, the screenshot is quite clear: if you are setting a GSX custom profile, the only thing missing is the last waypoint 3, the other will arrive from the Simconnect Navdata API, and will be different for each airplane so, as long the jetway behaves correctly, and you set the last waypoint height, you shouldn't have passengers with their heads sticking out (unless the jetway ceiling is unrealistically low).
That's for the MOVABLE part of the jetway.
If you want to see passengers continue walking in the STATIC part of the Jetway (be it a standard "JetwayLink" object or even a static mesh), you can create custom Pax waypoints in the GSX Profile.
When there's a jetway, custom Pax waypoints control paths AFTER passengers reached the Jetway root, because they would normally disappear if there were no custom waypoints. This is different than apron parking spots, where pax waypoints controls the whole path when passengers are not on the Stairs.
Pax waypoints can have an height as well so, if you have slanted STATIC mesh representing bridges after the Jetway root, you can have passengers walking through them, in 3D.