Actually it is never too late to introduce a female or a male gerbil to a friend. Gerbils will be happier and live longer lives if they are in pairs, trios, or 4-somes. Females are best in pairs, since they are the dominant sex and can have dominance issues in trios. Males can be kept in groups up to 7 or 8 gerbils depending on the amount of space you have them in.
I would recommend getting your gerbil a female friend, but you will have to use the split-cage method to introduce them. You do this by getting some hardware cloth mesh to split up a tank. Put one gerbil on each side and switch their sides 3 or 4 times a day. (Do not clean the tank completely for a while.. just spot clean) Switching their sides allows them to get used to each other's scent without harming each other, so they eventually both smell the same and consider each other clanmates. Ways to know your split-cage has worked is sleeping beside the mesh near each other, sleeping in each other's nests and trying to groom each other through the mesh.
You can find out more information about how to build a split cage and how to do it here:
Gerbils do not change sexes... That is the most ridiculous thing I've ever heard. People can sex them wrongly and mistake a female for a male or male for female, but really gerbils are very easy to sex.
A 10 gallon tank is big enough for two gerbils, but they would always be grateful for more space if you can allow it.
Keep in mind that two females are one of the hardest introductions, so the split-cage might take 1-3 weeks or even months. Don't give up though!