tmobile gateway restart

You asked. The easiest way I can suggest is a programmable "smart" plug that the gateway is powered through. Sure it is not a remote access solution but it would work. If you want to have the gateway reboot at a specific time of the day that would work. Since you cannot make a remote connection to the gateway from the internet some local client that can be used is necessary. 
Another option, sure maybe, is to call T-Mobile support and ask them to reboot the gateway which they can probably do for you.