The test you did was with no boost helping push the wastegate open, so either your wastegate is opening by itself (broken actuator), you have a large leak somewhere, or you have some major issue elsewhere (clogged cat, very low compression, etc...)
You can try wiring the wastegate shut and doing the same test again to rule out the wastegate actuator. Wrap stainless steel wire around the wastegate lever, loop it to a bolt hole and twist it down to keep it shut. If you're still only getting 8psi, you have a massive intake leak (likely) or a crack in your turbine housing (unlikely)
And yes, no gasket at the BPV would definitely result in a large leak.