You are able to cast an OpenGov vote from your multisig with Polkadot Multisig. Choose "Voting" from the left menu.
You can set:
- The Proposal number from the dropdown list
- AYE or NAY (note: abstaining is coming soon)
- The amount of tokens to vote with
- The conviction, i.e. from 1x to 6x, which will give you more votes, but lock your funds for a longer period of time.
Finally, you will be able to review your vote on the summary screen.