I don't know about the gate thing, but I did have this little gem of advice, lol...
Dreamer, my Spotted Saddle Horse, likes to stick his head through the bars of our portable panels. Then, he freaks the heck out because he cant remember how to pull his head out. He'll set back and shake his head and literally MOVE all the panels around. I've seen him turn our round pen into a cresent moon shape =/ its even worse if there is another horse out with him, because they'll bite his butt and kick him and he's too dumb to pull his head out, so he jerks his head UP and runs with the panels. VERY VERY DANGEROUS.
Needless to say, i dont put him in the roundpen unattended anymore.
I just wanted to let ya know about that. it has nothing to do with the gate, but could be a problem none-the-less =)
Congrats on the potential mustang purchase!!