FCOM DSC-22_30-40-90-20 "Engagement Conditions"
SRS GA engages when all of the following conditions are applicable:
‐ The flight crew sets at least one thrust lever to the TOGA detent
‐ The flaps lever is at least in position 1
‐ The aircraft is airborne, or on ground for less than 30 s.
SRS GA mode can be engaged regardless of the vertical mode (except SRS TO).
For reference of the ideas above.