After mounting early pressure the GForce went 7-0 ahead through a James Faaatuatu try converted by Scott Meakin.
Sam Whitehead reduced the deficit with a penalty but he limped off soon afterwards with a recurrence of his knee injury.
But Guernsey, who dominated territory throughout, then moved 8-7 ahead with a Matt Hema try which unluckily did not earn the extras with Glen Walters’ kick striking the post.
A converted Tony Borrow try gave GForce a 14-8 interval advantage, which was extended through another Meakin penalty.
Walters had their chances and missed a couple of penalty attempts, but they got right back into it when a catch and drive move resulted in Richard Hile going over.
Walters’ kick struck the cross bar this time and fell out rather than over.
At 16-17 and a temporary one-man advantage, Guernsey were in with a chance of a rare away win.
They pressed hard for a third try, but the only further scores were two Meakin penalties.
‘We didn’t deserve to lose that,’ said Guernsey RFC committee man Mike Marshall.
‘It was a good performance,’ he added.
Team: Cameron Craine, Bradley Choules, Layton Batiste, Lewis Hillier, Simi Sawene, Jason Batiste, Robin Le Cocq, Nick Merrien; Malcolm Barnes, Glen Walters, Matt Hema, Blair Campbell (captain), Ned Brown, Richard Hile, Sam Whitehead
Replacements: Simon Sharrott and Dave Davison