To help with putting this type of setup together we have put together the following pin diagram. We do our best to keep these drawings accurate, but we are still human and on occasion do make mistakes. If you find such an error, please let us know so we can fix it.
You can start with a prepared cable from ebay, such as those linked below, and then just add a DB9 Male to interface with the pi-repeater db9 female cable.
https://www.ebay.com/itm/Cable-63-Motorola-16-pin-Maxtrac-GM300-VHF-UHF-Repeater/350188820923 (3ft)
https://www.ebay.com/itm/Cable-66-Motorola-16-pin-Maxtrac-GM300-VHF-UHF-Repeater/120666387915 (6ft)