I found this 'pick n fly' from Naturals on Northern Parrots for the gerbils. It does also say for rabbits but i wouldn't feed the seeds only the grasses in it. The gerbils i think will love them because millet sprays are already their favourite. The millet sprays were on offer so i picked those up too, you can never have too much millet!
I do like giving seeded treats to the gerbils because in their natural environment that is what eat. They also eat plant roots and oats. I include these in their diet  3-4 times a week to make sure their diet is balanced and healthy.