Graphical applications which use GTK require additional libraries, environment configuration and interfaces to function correctly inside a snap. Follow the instructions for the version of GTK your application uses.
The gnome-3-28 extension only works with snaps that use base: core18. There are some tools that can help you if your snap doesn’t use base: core18, however, it is preferred to upgrade your snap to use base: core18.
Should there also be minimal example to define .desktop file integrated into the snapcraft.yaml. Gtk apps need to have a .desktop entry in order to run correctly in snap.
When using bases, snapcraft no longer support remote parts. You need to redefine the part. You can see below for an example. Going forward snapcraft will include an extension for “gnome” which provides what the remote part did, however that isn’t done yet.