From Turnpike Trusts on the Plymouth History encyclopedia site
"With Plymouth now reachable from Exeter, attention turned to the road
from Plymouth to Tavistock. This was made the responsibility of the
Tavistock Turnpike Trust by Act of Parliament in 1761 and included a
"branch" from Manadon Gate to Stoke Damerel. Tolls were to be collected
at three places, Knackersnowle (the present-day Crownhill), Horrabridge
and the *Abbey Gate Bridge in Tavistock*."
This doesn't completely answer your question, but it was clearly a toll
gate for the turnpike. Perhaps the job of toll house keeper wasn't
onerous and your ancestor fullfilled the role as well as being a miller.
The mill was presumably run by a water wheel - next to the river or canal.