When I built 2.8 here a month ago, (not currently on this new install) I didn't need to use a script to export & launch.
So in that case it was simple to create a .desktop just for 2.8 though you could use the script if need be on the Exec=, if so see note
Are you sure you need the launch script?
Anyway laid out basic here
For the .desktop I'll repeat here, though I used a slightly different Exec=, TryExec= red, then you show in your script
Then just dragged it from ~/.local/share/applications/ on to the launcher, no issues
Comment=Create images and edit photographs
The Icon= presumes another package version of gimp installed, otherwise needs to be full-pathed to an icon
Just to note - even when I make a .desktop that uses an Exec=script I always end with a <space>%U or %F or %f. This will allow the .desktop to be seen in the r. click context menu, %U should be ok