At the moment, the data type in Google Sheets is used to mark a column as dimension or metric, i.e. numerical values are metrics and other values are dimensions (https://supermetrics.zendesk.com/hc/en-us/requests/30218?page=1)
This is not helpful and it’s not possible in all cases to change the data type in Google Sheets without causing problems with formulas.
All columns should be available as metrics and dimensions