Prime gears are tekgrams, so like vanilla tek armors, you need to learn the tekgrams in order to equip one. To learn the tekgrams, you have to kill the alpha tribesmen of same armor in order to learn the full armor set tekgram.

In this example, to learn prime cloth armor tekgram, you have to kill an alpha yellow tribesman.
Red for chitin
Blue for scuba
Orange for flak
and so on.

Wich armors you can't wear. If you mean Prime Tek, then you need to learn first all vanilla tek armor engrams to can wear it. Except that have and level requirements. For all other prime armors you need to kill alpha tribesmans to learn prime armors and afetr that you can wear it

i have killed the alphas but its not unlocking the engrams