A header tank does not signify a gravity system. The gravity part refers to the way the hot water is circulated to the cylinder. No pump is used on that circuit and it relies on thermo syphoning from the boiler. Normally there are 4 pipes from the boiler and the HW circuit is in 28mm pipe.
If you post a pic of the relevant part of the system it save s a lot of confusion caused by using incorrect terms.
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