Legal Question in Real Estate Law in California

Did this happen suddenly, without any prior discussion or dispute about your use of the gate? I'd say if it did, you should discuss with the neighbor to learn his motivation. If there was already some kind of discussion or dispute going on, the LawGuru lawyers would want to know more about it before offering much advice.

One analytical problem is that the fence seems to be on, or very close to, the boundary line. This raises a question as to whose fence it is, which in turn determines whether the sealing of the gate is a trespass or not. It is only a trespass if the fence is (a) on your side of the boundary, and (b) belongs entirely to you. Even if you paid for it, the fence isn't necessarily 100% yours if it is a true boundary fence.

A larger issue may be what use was being made of the gate. Sounds like maybe you were using it to get to and from your property via your neighbor's driveway. Unless you have an easement to do this, this would sound like trespassing.

So, based on the limited facts, the neightbor might have committed a trespass of a rather minor nature to your fence, to prevent you from committing repeated semi-minor trespasses to his land. However, one cannot be sure, not knowing all the facts about ownership of the fence, existence of easements, etc. etc.