your choices as above are
GD- mechanical, ugly, basic, stepped drops, reliable and easy for owner to service and maintain.
KSi7- “infinite” adjustment, mixed reliability reports with plenty of “needed sending back and rebuilding from new” posted on here, and appears a change of importer/distributor means you cant find em. Ugly/clumsy with the external piggyback cyclinder.
X-fusion- “infinite” adjustment, some largely positive reviews but still a bit of an unknown quantity.
How much drop do you need, my GD drops to 4″ which is more than enough for arse-on-back-tyre shenanigans.
I assume fox/rockshox would rather slightly limit their market not catering for 27.2 than make a post more unreliable by slimming everything down too much (why KS needed to move half the stuff externally and not many hydraulic/pneumatic post makers have tried to go that small?).