Most lumber will move quite a bit outdoors. Screws don't restrict movement like glue or adhesives do. Restricted movement often leads to splitting. You are better off not to glue something like a frame added around individual T&G boards, for example. If the gate is made more like a door, structural frame with M&T and loose panels, you can use glue in the M&T joint but you need to leave room around the panel to move. I've built many shed, garage, barn doors and gates. On one set of doors, I used 1x6 T&G boards with a 1x4 perimeter frame that was screwed and glued to the surface of those boards using Gorilla glue. Several of the 1x6 split because of the glue. A lesson learned the hard way. I have a similar door on one of my garden sheds. The bottom member of the frame is rotted. Having no glue, it's easy to remove the screws and fit a replacement part, which I couldn't do very easily if glued. Nothing outside lasts forever. Plan for future repair.