Yeeeeees, all that luggage to get into central Manchester,Is there a long stay car park there to leave the car? Then on to the train, off again at Euston, nice 'walk' with all the luggage to the tube, on and off the tube up and down escalators with half of London barging you out of the way, luggage back on the LGW express and off again, nice long walk to check in, queue up and finally through security.
Alternatively, check in at MAN, luggage too, security, wander up to the gate, nice flight down to LGW, stroll to the departure lounge, board flight to paradise...etc etc.
I get it. If and its a big if, you are travelling light, point to point, MAN to LGW then maybe the train is a good bet. But with a family going on your jollies with all the parafanalia that goes with it, a direct connecting flight is my preference.