User blog comment:Thetwindler/Massive Supernatural Plothole/@comment-3169324-20180509223229

The angels know very well what happened to Castiel when he consumed souls - he went insane. Regardless, it was said to be very dangerous, since the vessel might not be able to withstand it. Castiel's vessel is unique in that regard, as it appears to be nearly indestructible.

Gabriel is weak. I doubt he can withstand the combined assault of every demon in Hell once they realize he's trying to steal the souls there. He also can't just walk into Heaven. The angels don't sell/use the souls in Heaven, no matter what.