I just did my pop out gaskets and the vertical one is not that difficult, but the glass needs to come out. You’re not replacing the “C” shaped gasket that fills out the rest of the frame?
The glass is held into the metal channel by a small cotter pin at the top, so peel back the existing gasket to access that. Or you can remove the metal channel with the glass, but I would recommend taking the glass out first, then the channel and gasket. Not hard, but definitely don’t try and pull the glass out without removing the cotter pin.