First and foremost! :You want to be very careful you are now sending company mail that belongs to your company from your company owned and operated email to an external source. In most states it is considered Corporate Theft and it’s a felony.
Depending on the version there should be a MORE (8) and a TOOL (7/6) button that will lead you to the RULE. Once in the rules you should be able to create one that says something like:
All mail that comes in should be forwarded to your gmail account.