Would suggest perhaps a look at the Dorset militia rolls. The militia was a compulsory force.

Dorset had three regiments: Dorsetshire Militia, East Local Militia and West Local Militia. The latter two raised in 1808.

As the militia was under the County Lieutenants, the muster rolls may be held in the County Archives. The National Archives also has militia records but catalogue is not online.

As Gale apparently enlisted for limited service, he may have volunteered from the militia to serve in the regulars.