The Northern Irish shot-stopper has joined the Clarets until the end of the season.
Gartside (pictured), 19, joined the Hornets’ academy in 2014 and signed his first professional contract two years later.
Miles makes the permanent switch to City from league rivals Hemel Hempstead Town.
Miles joined the Tudors at the beginning of this season after his contract with Lincoln City ran out.
The 22-year-old had a four-year scholarship with West Ham United before signing for Concord Rangers - under the management of Danny Cowley - and has since followed Cowley to Braintree Town and then Sincil Bank with Lincoln City in the summer of 2016.
However, the talented midfielder struggled with an injury during pre-season and thus was unable to regularly break into the first-team, meaning loan moves to Boston United and Concord Rangers were the only chance for Miles to achieve regular football.
The creative midfielder signed for Hemel Hempstead in June 2017 but now joins the Clarets and is in contention to make his debut for City as they travel to Weston-super-Mare tomorrow.