It depends on how tight you want to make it. I think I used to do them at about 3 g, but it's a while ago, memories fade (3g at the bottom, over the top the gs would normally be a little less than one.) You could do one at 2 g, but with a low pitch rate, you are in danger of running out of airspeed before getting around the top of the manoeuvre. The faster your steed is and more power it has, the more leeway you have for doing a low g gentle barrel roll.