The stones have no passive abilities in the MCU and need to be activated so that wouldn't be enough. But he'd probably just destroy the portal with the reality gem. Also I think the writers have made comments that suggesr the group wouldn't have beaten him even if they removed the gauntlet and Strange would've known that.
EDIT: If the question was why didn't they cut his hand off while he was held in place then there's really no answer unless Strange has a long term plan. Although it might just not work, portal chop worked on Cull but he's less durable.