I Alfined my Dean Ti hardtail a while back. It worked fine in a spinny sort of way. The hub will take grunting in high gears but seems to rob the bike of a little of the crisp pickup it has in singlespeed guise, so most of the time I run it SS. I can make the change from SS to Alfine in under 15 minutes though for when I feel I might want gears.
The Alfine went through the monsoon that was the 2008 Dusk til Dawn without missing a beat.
Alfine is great, but you'll need a chain tensioner to run one on a G Spot (as you probably already know) because despite the BB main pivot it only has conventional vertical dropouts…
Test rode a G Spot a few years ago myself, found it to be susceptible to pedal induced bob more than any other bike I'd ever ridden! I'd suggest that due to the Alfine essentially requiring you to get out of the saddle and crank a bit harder (not as hard as on an SS) due to lack of such low gears as with a full 27 speed setup, it is far from an ideal combo though! Alfine hubs work superb on hardtails, I'd consider running one on a short travel full sus that pedals efficiently, but due to weight and increased pedalling torque required wouldn't want to run one on a long travel bike that bobs a lot.
i currently run a 36 front ring with alfine on one of my whyte prestons and have no need or desire for a lower gear,, even the top is just about acceptable just need to spin a bit to keep up on the road,,
the steps between each gear are bigger than a normal setup but riding in the deep mud at the weekend it was great when stalled out in the mud to select a gear without having to spin the pedals