If you want to open text files in Gvim via file manager, open the ~/.local/share/applications/mimeapps.list and add the following to the [Added Associations] and [Default Applications] sections:
application/x-perl=gvim.desktop
text/plain=gvim.desktop
text/x-chdr=gvim.desktop
text/x-csrc=gvim.desktop
text/x-dtd=gvim.desktop
text/x-java=gvim.desktop
text/mathml=gvim.desktop
text/x-python=gvim.desktop
text/x-sql=gvim.desktop
text/x-tex=gvim.desktop
