Haven't tried the bitcoin-core-gui but I am sure that you can configure it to use TOR Socks5 proxy. Use 127.0.0.1:9050 and check torrc file in "Browser/TorBrowser/Data/Tor.
I think that you should remove all lines that contain ReachableAddresses and then restart tor. That's all I found on the internet right now. Test it and post your results.