Probably more accurate to say Stormwall and Galleon to a lesser extent than all of them.
This is a very interesting thread on the PPS forums:
http://privateerpressforums.com/show...On-Gargantuans
Towards the end of the first page, someone breaks down the number of Gargossals used in the 320 lists at the WTC:
Gargossal:Player ratios
Stormwall:Cygnar 25:21
Galleon:Mercs 4: 4
Mammoth:Skorne 6:11
Conquest:Khador 6:12
Hyperion:Retribution 5:11
Judicator:Protectorate 7:19
Woldwrath:Circle 5:20
Kraken:Cryx 6:25
Archangel:Legion 1:24
Mountain King:Trollbloods 0:11
Of the 21 Cygnar players, there are 25 instances of Stormwall. That means there are more Stormwalls being used than Cygnar players.