I recently stayed at Anantara Lawana Koh Samui. The experience was good, I booked a pool access villa and was given a nice +2 upgrade to a seaview pool villa.
Overall service in the resort was excellent.
My impression, based on my limited experience with the GHA program, is that the GHA hotels do try to give an upgrade when available, and this is a very big plus for me, so I will consider sticking with this program as long as I can.

However, I do agree that the D$ is difficult to spend. I usually book non-refundable rates to save money, and most GHA hotels require prepayment by credit card, and this makes it impossible to use D$ for the room rate.
I'm usually not interested in spa or dining in the resort, so options to use the D$ are limited.
GHA really needs to do something about this - to allow us to use D$ for prepaid room rates, or if that it not possible, to allow us to use D$ as refunds to credit card upon checkout.