Manchester United winger Nani has arrived in Istanbul to complete a move to Fenerbahce.
Liverpool are set to confirm the appointment of Sean O'Driscoll as assistant manager on Monday.
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.