I want to add that the desktop extensions are now the recommended way to create a snap for desktop applications. The snapcraft-desktop-helpers project should only be used for a few very specific situations such as 32-bit applications and legacy GTK+ 2 applications.
@adnan338 for your application I would recommend the gnome-3-28 extension. This one is specifically built for GTK 3 applications. It automatically adds the desktop launcher to your snap and includes an number of improvements to make your snap smaller, start quicker, and make it use the system themes.
See GTK 3 applications for a complete tutorial on how to create a snap for a GTK 3 application.
Edit: I didn’t see that you require core20. A gnome extensions for core20 is in the works, but not released yet.