Not sure whom to choose, in modern cricket not sure if it is possible to play four fast bolwers with some flat pitches and over rates to contend with. That being said Garner at times played the role of a spinner keeping it tight with his yorkers and rib ticklers. All of the bowlers contended will with flat wickets and turning tracks especially Marshall, Holding and the aforementioned Garner. The effect of Sobers though is greater felt as a seamer than a spinner, especially as an attacking option, but could have kept it tight as a spinner as well and rip it with his wrist spin option.
Gibbs is also under rated and a notorious spinner of the ball, if he had the conditions of Laker and some of the other early finger spinners no doubt would have had similar results, but as it is he was just or more succesful as the Indian spinners and to boot the first spinner to 300 test wickets is no mean feat.
Will think about it a bit more before I vote, Garner's average and s/r vs Gibbs variety. Both were, to boot excellent fielders aswell. Tough decision.