bulkhead DOH has contact gates and remote stands. For contact gates you may arrive from a 'secure' origin and will be directed directly into the departure area. I think BHX is often on the secure list. Otherwise you have to follow the signs to Transfer and go through Security to enter the departure area. For remote gates a dedicated J bus will take you to the transfer area. Since you appear to already have your onward boarding pass just go through security and enter the departure area. There are dedicated F and J Fasttrack lanes in the middle of the security area.
Departure is always from a gate but if the aircraft is remotely parked you will be taken from a so called Bus gate to the aircraft by a dedicated J bus. Bus gates are one floor below the main concourse. C-gates halfway down and D and E at the end of the C wing. Bus gates are numbered 20-30 for C, D and E wings.