Prybar. 2x4 against the frame and hit with a hammer. It is not the gasket that is holding them in, it is the plastic sleeve on the studs that is siezed in the hole in the doors which it slides into.
The safest (but most time consuming way) to remove them properly is to remove the door panel (carefully, they ar fragile, especially be careful when tightening the screws on reinstall not to make them too tight and crack it, use small washers on the screws if it is already cracked) and put two nuts on the bottom of the studs, and use the top nut (whilst keeping the lower nut tight) to unscrew the stud from the bottom of the door top. Once both are unscrewed, the door top will be loose and will be able to be removed with relative ease. Then you can [carefully] hammer the studs down through the hole and out the bottom. This assumes there is enough thread to get two nuts on. Their usually is, but one of mine, there was not.
Once they are out, clean up the holes where the studs go into the doors and run some sandpaper around the plastic sleeves to slightly reduce their size so the door tops can come off by hand again. I put oil on them as well before reinstalling.
*not legal advice*