I have several freeware stuff (LEPA, LIML, EDDH) there and with none of them a proper pushback is possible. Moreover, it happens that I cannot call the deboarding when parked at a clearly marked park position (the menu shows only parking positions, runways and gates to get to).
It would have been more helpful if you sent a log with you parked at one of *these* airports, instead at EDDM, unless that one is having problems too. Please confirm if this is the case.
If there's a disagreement between what you see and what GSX works on, like misplaced parking positions, etc. it's usually caused by an AFCAD conflict.
Sometimes, the AFCAD conflict is already happening within FSX itself, so you'll see misplaced parkings even with positioning the airplane using the standard "Go To Airport" FSX menu command. In this case, there's nothing that GSX can do, if FSX itself shows a conflict, GSX surely cannot fix it, you must the remove one of the conflict sceneries, or work with the Scenery Area priority to sort it out.
In other cases, it's possible that FSX has managed to support the AFCAD configuration, but GSX is not.
There are some configurations that GSX simply doesn't support, for example airports that are not entirely self-contained. In order to be considered usable by GSX, and airport must have at least one parking and a taxiway. It's possible that some airports, especially freeware ones, don't include everything, but just "enhance" what's already available in the FSX stock AFCAD, and build on top of it with a separate .BGL that only includes their additions.
In this case, if the additions are enough to consider the airport "complete", and the enhancing AFCAD is on an higher layer in the Scenery Library, GSX will use ONLY the enhancing AFCAD, but no data from the stock one will used by GSX, so you'll see only the additions in GSX.
If, instead, the enhancing AFCAD is not considered complete by GSX, for example because it has only new taxiways but no parkings, GSX will use ONLY the stock AFCAD, so you will be missing all the additions.
Basically, while FSX has the ability to "merge" data coming from different .BGLs of the same airport, GSX will only use one at a time. We used this approach because it's far simpler to support, and all default airports are entirely self-contained, and most of the 3rd party scenery addons are also self-contained in a single file. Some developers use different variations, but the relevant data (parkings and taxiways) it's usually used in a single file at a time.
With a log file parked in the positions of an airport that is giving you problems, it would be possible to be more sure about what your issue really is.