Adverse possession is not easy to get. Most of them end up in court for a long time and you have to prove there was no attempt to gain use of the land by the other. The time line is different for each state I think. Here in WI I beleive it is 20 years. I have seen a few of these disputed and I can't remember any of them working in the favor of the owner claiming adverse possession.
Good luck. It may be cheaper to move your gate back, I'm not sure of you situation though.
