I don't think it's a question of how Gbuy might compare to Paypal but what improvements can they think of. If they have considerable improvements over ...
For example this is what Paypal take for about all transactions:
1.9% + $0.30 USD per transaction.
If Google can do better than that (since it might be a bigger corporation) then I think Google might have a chance.

And one more thing that JV mentioned here... what about Adsense? Doesn't Google have the power to pay perhaps millions of Adsense users with Gbuy (or only Gbuy) then I'm sure many people might start using Gbuy for sure, since they already get money into their Gbuy account.