No GUI found so far. I made the wallet work with command lines, but I haven't been able to setup the masternode as I don't know how to run those commands without the GUI.
You can start masternode in local wallet using command:
sub1x-cli masternode start-alias lock_wallet alias_name
where lock_wallet is boolean: true - lock wallet after starting masternode, false - not lock.
alias_name - name of alias, that you have specified in masternode.conf
for example:
sub1x-cli masternode start-alias false mn1