[Michael Cule]

This comes up regularly (checking back I see the last time I posted something about it was just over a year ago) and I'll give you my current thoughts on the matter as I always do...

It depends on how much work you want to do and how much of the individual flavour of Harn is important to you. There's a lot of systems now available for GURPS which can help you but what you choose to work on is very much a matter of taste.

I've changed my mind slightly over the past year and I'd now see the way to go with magic and miracles would be to give the priests a variant of Ritual Path Magic with each deity having their own path skill. This would mean slower but more powerful magic which would have to be prepared beforehand to be of use in combat (so your priests of Agrik would bless weapons to burst into flames in lengthy rituals rather than calling it up on the spur of the moment). You should also allow Priests and devout worshippers (a Perk) access to Divine Intervention: I like the model which requires sacrifice of saved Character Points to attract divine attention.

I would give the Shek-Pavar Realm-based Syntactic magic with the ability to have 'rotes' or pre-generated fixed effect spells which cost a Perk point. They would also have a penalty to using spells of opposing convocations until they made Grey Mage.

Neither of these is the easy option. If you want easy then preserve the main magic rules and give the Priests Ritual Magic and make the convocations of the Shek-Pavar Magical Styles.

I hope that's some help: the 3e adaptation is good as far as it goes but is very old now. (As am I!)