Probably not going to happen until the next release, when there's a chance to get them married to each other (as the eligible age span will be increased to + 50% of the younger one's age, rather than being fixed to 10 years).
Also, there are some significant bugs regarding Gremlins:
- The only way to make Gremlin sized clothes is to have a Gremlin make them. They're half the size of Kobolds, which are the next smallest. However, my last Gremlin died of stress before applying for citizenship, probably due to a large extent because of being butt naked (since then I've changed the Gremlin raws to make the Kobold sized, but haven't gotten a new one).
- DF does not allow you to assign labors for Gremlins, nor view their thoughts. DT can handle the former, but it's rather annoying to being told that your Gremlin baron has made a demand, but not being able to see what it is (that has happened to me, some time ago).
- Probably a bunch of others I can't remember off the top of my head: the whole animal people from the wild functionality (which applies only to Gremlins in vanilla) is a bug fest that's probably not going to be addressed until animal people get proper tribes.
I've tried in the past to collect Gremlins to get three breeding pairs (I think only two are required, actually) for a self sustained fortress, but haven't managed to collect more than 3 (none forming a pair, of course) in any fortress.
Also note that due to their size, they can't wield any weapons except knifes, and you'd have to liberate those from Kobolds, and it's a challenge in itself to get a reasonable number of knives from them.
Otherwise, it's correct that Gremlins eventually apply for citizenship. Before that happen they typically change status from Stray Gremlin to Stray Gremlin Hunter.