"Rust Buster," a citric acid-based penetrating liquid also works well on reluctant fasteners. I'm a huge fan of Kroil but IMHO, Rust Buster is what I'd use first on a fractious GP.
I worked in the appliance repair industry for about 15 years and have used this stuff hundreds of times on rusted, ancient appliances to successfully remove seized fastners.
It's sold in a 'zoom' spout dispenser bottle and, in the past, I've bought it at appliance parts distributors like Johnstone Supply.
To get some, go here (
www.johnstonesupply.com) and keyword search on "Rust Buster." I think you can even find a 'location near you' through their website.
For the DIYer, I'd be willing to bet that a strong mix of MBZ citric acid cooling system flush and water, applied to the outside of the GP area would accomplish the same thing as rust buster.