Originally Posted by
bisonrav
Gatwick Express -> Victoria -> Euston -> Piccadilly.
I agree with train but personally, I'd do Thameslink from Gatwick to St Pancras, walk to Euston (10 minutes) and train as there is really no point multiplying changes and peak time tube (yikes) to take the Gatwick Express which is not express at all and arrives in Victoria which is not helpful to Manchester. LGW to St Pancras takes about 45 minutes and should run every 15-20 minutes nonstop.