Bring the gammon to room temperature and remove all outer casings including string. Boil the kettle.
Heat 250 millilitres (1 cup) ginger beer in the microwave until hot. Add 250 millilitres (1 cup) boiling water. Add the 4 rooibos teabags and steep for at least 10 minutes. Remove and discard teabags.
Place roughly chopped celery, carrots, and onion at the bottom of the Instant Pot. Position gammon on top of the vegetables.
Add bay leaves, allspice berries, peppercorns, and cloves. Pour the rooibos tea mixture and remaining 500 millilitres ginger beer over the gammon.
Seal the Instant Pot lid and adjust the vent to sealing position. Set to manual pressure on HIGH. Cook for 15 minutes per 500 grams of gammon (a 2-kilogram gammon cooks for 60 minutes).
When cooking time finishes, allow natural pressure release (approximately 15-20 minutes). Do not quick release.
Remove gammon carefully and set aside. Strain the cooking liquid through a fine-mesh sieve, discarding solids. Reserve liquid.