Please be aware that although this is an easy way to fix the problem, it has the drawback of making you responsible for your launcher. If the way to start Monero GUI wallet will ever change, you will be the one to update it.
If you’re using xfce, already installed the wallet, and don’t want to start from scratch, you can add the launcher yourself. Just create a file with fitting name in ~/.local/share/applications/ (in the app qube). If this folder is empty for you or doesn’t exist, you can search for examples in /usr/share/applications/.
In general, a launcher only requires Name and Exec fields to function. Everything else is just for your convenience.
Consult monero documentation to figure out how to start the GUI wallet from command line (you will need this for “Exec”).
It shouldn’t be a problem for your use case, but keep in mind that launchers in ~/.local/share/applications/ are only available to that specific user. To add a launcher system-wide, you need to save it to /usr/share/applications/ in the template.
After adding the launcher, it will become available in the applications list in qube’s settings. You may need to refresh applications for it to show up though.
p.s. There is a guide for that, with links for further reading.