Hi,
The best enrichment I find is basically lots of household cardboard, paper and tissue and hay and soft straw. The best toys are the ones they destroy in 30 seconds. If you can find any of those hanging toys with a kind of skewer, you can also thread pieces of cardboard onto them for her to chew. As long as she has wood available in her enclosure, her teeth should be fine (assuming there are no medical issues). If you're comfortable posting a photo of her enclosure people might have other ideas. Some female gerbils can be quite easily bored!
I don't want to necessarily contradict what your shelter said because obviously they know her exact history, but I will say that shelters often don't have gerbil-specific knowledge and are often operating on assumed principles like "she's been in fights before therefore she doesn't get along with other gerbils". It's quite rare to find a gerbil who really cannot live with others and most of the time when introductions end in fights, it's because they weren't done properly or safely.