If I buy South Gambit, am I missing anything...

[DrunkenLuftwaffe 10]

...by not buying Knights of The Sea?

[Schrullenhaft 73]

I don't have a list of the units/platforms, but I would guess that some of the Russian, British and German platforms might NOT be in 'South Gambit'.

[Battlebug 10]

I got both a few days ago. It looks like all the units from KOTS are in SG. Knights does seem to have a lot more missions than Gambit though, and with the exeption of the tutorial those are all just in one of the games.