for the forces, well join the medical battalions via regular commission for medical professionals (physicians and dentists). Afterwards you would need to take approval from the GHQ for specialization in a particular field and doctors go within and outside pakistan for these. You just can not do specialization and then join pak army.
anyways, regarding the demand and supply gap, well all fields are always in need of a good medic but the fields with least amount of doctors are Orthopedic surgeons with specialization in prosthetics and implants, neurologist with special interest in sleep and pain medicine, Plastic surgeons, vascular surgeons, rheumatologist and finally ENT specialist with specialization in cochlear implants.
You would be surprised that some of the fields mentioned above have less than a dozen specialist in pakistan but all of these will require a basic specialization (4-5 years) followed by a post specialization (3-5 years). So in short you are easily looking at a decade.
Think wisely and as per your family and financial requirements.