The main GC I work with performs maintenance (of all types) at over 160 gated communities in So Cal. I'm often called in to perform electrical repairs on gates, even before the "gate guys" are called in.
Although I usually let the gate guys handle the mechanical side of things, just performing the electrical work is a pretty good market - albiet inconsistant (but we're all used to that by now, aren't we?).
I'd say that, if you really want to get into it and perform all the repairs (electrical and mechanical), you should probably consider it a full time venture, and be prepared to be on call 24/7. And also, be prepared for some irate homowners and property managers, even if the problems are in no way your fault (we're used to that by now too, I imagine).
The most calls I get are around the holidays, when the communities are putting up all the (badly installed) holiday lights at and around their gated entrances. This tends to be the acid test for an iffy power disribution system.
I don't know if I'd recommend this venture to someone that doesn't like to be at the mercy of the maintenance gods, but there is definately some money in it, especially since most new communities are being built with gates (out here anyway).
As for the actual work, it's pretty easy and has few components to troubleshoot - easy to narrow down a problem, and easy to fix if you have a well stocked truck.