Is it possible to enter messages as data into Google sheets via Workflow? For example, inside a slack channel, I want to move messages or responses into an external Google Sheet.

Gaurav is correct, although you may need some additional help.

• If you are trying to move messages/items contained within a workflow, then yes - just use the included Google Sheets connectors.
• If you want message data from existing messages (or messages posted by users) then you will need to build and install a custom workflow step that reads the message text, then you can use that data in your GSheet step to add to the sheet.