@Iceache
Unfortunately there is so much conflicting info on the internet and only a handful of sites that give good advice, which is why I resorted to writing my own. The information that you have been given, that it's very unlikely you can bond a female over 6 months is incorrect. I have bonded different ages and sexes of Gerbils, I've helped the public with their bonds and am in touch with other rescues that regularly bond. Gerbils are classed as exotic pets, in the wild they have have complex hierarchical structures, and in some cases, yes, they will not bond. That's true with any other animal. It is also true that if Gerbils do not bond they will fight (which is why you need to be on hand with gardening gloves when you take away the split divider), and in very rare instances they will fight to the death. That can unfortunately happen, at any time, with any bonded pair, but is rare. So the advice you have looked at has focused on the worst case scenario, when in reality every Gerbil can find a partner, even if it doesn't work out the first time. In my experience it's 70/30 in favour, with no difference between males and females. This is why following the split cage method is vital as it gives the best chance of success.
You wont' end up with 2 singles. If you adopt through a Rescue they will always stipulate that you return the Gerbil if it doesn't work out. No rescue is going to hand over a Gerbil to remain single for the rest of it's life and like wise your remaining Gerbil still needs a partner.
Pups under 8 weeks - they haven't developed adult territorial traits so they can be bonded with any adult with very little split caging and guaranteed success. Personally I don't like this combo, as when the youngsters grow up they can, in some instances, try and become dominant and push your original Gerbil out. If your remaining Gerbil is young enough (under 8 months) so there isn't a large age gap, you could try with one under 8 week pup. So either try an adult to adult bond, or 1 x under 8 weeks old pup.
I'm not sure if your remaining Gerbil is male or female. From your original post your Gerbil that passed is male, but your remaining Gerbil is female?
You can create a split cage in your existing tank, providing you have the right sized housing. If your housing is too small (i.e. small Pets at Home style Gerbilarium's) and you don't want to spend too much money you can pick up second hand aquariums and adapt them very cheaply. Making a split just needs wood strips and some mouse wire so not expensive.
Everything you need to know is here. I show how to source suitable tanks, where to find all the items you need for setup, how to bond and I've made it very easy with links too Gerbil Care – Promoting best practice when it comes to gerbil care and welfare