I'd qualify that by saying 'longest platform on the domestic rail network'. Eurotunnel Folkestone Terminal has 8 public platforms all with a platform length of 2595ft, easily beating Gloucester's 1977ft. Also the platforms at Cheriton are all single numbered ones - Gloucester's continuous face is two numbered platforms with dead space in between where, due to a crossover and signalling, passenger trains don't stop.
Anoraky question: is that likely to change when platform 20 at Waterloo (one of the former Eurostar platforms) comes back into use for domestic services next spring...?