I have the A6D, the procedure is the same. Follow the flow chart in the manual and pages 15-16, it is not complicated.
GTP-A8 manual1. Plug in.
2. Press stop until "program select NiCd" appears.
3. Press enter to get "NiCd charge man or aut"
4. Press enter again and the "cur limit" xxA will blink.
5. Press "<" and ">" at the same time and the top line will change between "man" and "aut".
6. You still have to set the max current charge to avoid possibly damaging the battery before you proceed.