If you like to forward a privileged port like 902 you need to be root.
Bug ID 1260685 was about this problem but the solution to show the error is not good enough.
gSTM should just ask for the root password and don't show any error at all.
Otherwise it's not really possible to forward these ports. Sure you can run gSTM as root but then you don't see you other tunnels because of the user change. Always, starting gSTM as root isn't comfortable because you have to enter the root password.