Is in बन्ध - bhanda (’binding’)

婆字門，諸法婆字離故

麼字門一切法縛不可得故

入婆字門，悟一切法離繫縛故 (Xuanzang 100,000)

入婆字門，悟一切法離縛解故

'BA' is a gateway because all dharmas are free of mental bindings (bhanda बन्ध)

Alt. translations

BA indicates that the Bonds have departed from all dharmas [Conze]

BA is an opening because of the bindings (bandha) of all mental phenomena are undone. Jayarava

Notes:

四縛 The four bandhana, or bonds. are: Desire, resentment, heretical morality, and egoism; or Desire, existence, ignorance, and unenlightened views.