Depends upon which engine size you are discussing. In this case it sounds like you are talking about the 61 engines, and as w8ye said, there isn't much between them. The Pro is a bit stronger at high revs & the GP is a little happier pulling a big prop.

Being a plain-bearing engine, the GP potentially has a longer lifetime.

However, they are both good (quite a bit better than their respective OS competitors), so you can't lose either way.

In the case of the smaller sizes, the Pro series engines are seriously more powerfull than the respective GP alternatives.