I think what's being drived at by Dave: If it's the external one then most TM GBBs aren't designed with a 'silent fill' o-ring, and that's why they spurt gas everywhere. I believe this isn't a fully rectifiable problem in all of their guns as adding your own o-ring on some brings with it some fulling issues (i.e. it not depressing the valve enough to fill the magazine fully). Not sure if that group includes the Hi-Capas though, so you may be able to add your own if it's annoying you.
If it's the internal o-ring from the valve (i.e. the one you obtain threw unscrewing the brass from the aluminium tube) then yes, you very much need that like Monty says.