6 hours is a bit much for Gatwick-Manchester. I live in Croydon, 15 mins north of Gatwick by train and travelled by rail this morning to Manchester, with luggage. It took 3h50 by train today, Sunday - ie with trains at least 30 minutes slower than Mon-Sat, from East Croydon station to Manchester Piccadilly. Trains from places like Wilmslow, Macclesfield or Stockport will be 10 or 20 mins faster. A train on Monday from Gatwick to central Manchester should be about 3h30
Anything more than 4h by train from Gatwick train station to central Manchester by rail suggests something going badly wrong. 6h would allow for an additional long lunch in a restaurant en route