Please turn this into a non-Whonix specific question as per https://www.whonix.org/wiki/Free_Support_Principle.
That is something along the lines "how to configure Monero GUI to use a socks4a or better socks5 proxy? Or “how to configure Monero GUI to configure Tor?”
Once you figured that out, please post the answer/reference here for review for Whonix integration purposes. (Sometimes it leads to Tor over Tor but I haven’t looked into Monero GUI.)