Cardiff midfielder Aron Gunnarsson has signed a new two-year contract extension with the club.
Celtic have signed defender Dedryck Boyata from Manchester City on a four-year contract.
The 27-year-old, who made seven appearances for the Hornets between 2010 and 2012, will provide competition for fellow keepers Heurelho Gomes and Jonathan Bond at Vicarage Road.
Gilmartin initially joined Watford from Walsall and has also had spells with Yeovil, Crawley and Plymouth.
He has spent the last year at Irish club St Patrick's Athletic.