The easiest way of changing a formula to produce the negative of the stored value is to put a minus sign in front of the ADGET function.
= - ADGET('gldata.M',$B12)

Whenever you make a change to the formula, you have to press Enter to accept this change. This behaviour is probably a little different than Excel.

If you are using the drag/drop method of creating statements, you can have the Financial Reporter do this for you automatically by editing the account Range from the View Account Groups screen, and select the Revenue (or Liability) checkbox.

You should also know that any changes made to a row that is a "Generated Row" are lost if the statement is a ".SmartSheet" and ".AutoUpdate" is enabled. These rows are deleted and re-created when the statement is refreshed, to pick up any new accounts added to the Ledger. If you do not want this behaviour, just clear the contents of cell A1 to turn off the .SmartSheet - then you'll need to make manual adjustments to the Statement if you add accounts.