I found a way to garrison my units into siege rams and towers through a gate. This works with horizontal and vertical placed gates only.
Here’s how to reproduce the problem:
Place a gate horizontally or vertically. On other side of the gate place unit and other side a siege tower.
Move siege tower between gate’s side part and actual gate part as close as you can get (See picture below). Now choose your unit and click siege tower to garrison your unit through the gate. You might want to wall in siege tower or units so they won’t try to walk around just to make it easier. Watch short linked video for better explanation. https://youtu.be/8ZrC0bii3a8
Of course, normally it isn’t that much of a problem cause buildings take long to build and rarely would you make a tower at their wall just to evacuate units out of someone’s base. With garrisonable units though this might have some bad effects even by getting units inside the enemy base if you happened to get ram inside, although the chance of getting just a ram inside someones walls is pretty low, without destroying the walls. But not a good thing obviously. However if anything can be done about it, well I’ll leave that to devs. >:D