The best advice any new user of the G2 can get IMHO apart from the obvious make sure you read the manual.
You say you are used to using the AF on a Nikon F4 and half press focus lock, now the G2 does this of course but don't use it. Follow the technique used in the link Paddy posted and use the "rear" AF lock button and I guarantee you will get much better results once it is second nature to you, keep that thumb hovering on that button and you won't go wrong with the G2 - I feel that those who never master the focus on the G2 and complain about getting out of focus pictures too often are those that don't use the rear AF lock button.
JM2C 🙂