That is the bigger and newer hull GTS in the picture so it uses the 155mm pump, I have the same ski but it was rebadged to GTI in my 2003.
In 2001 Seadoo made two different GTS's just to confuse everyone. The small GTS that is the old hull from back in 1991 with a 720 and a 140mm pump and the new bigger GTS that you posted a picture of with the 720 and 155 mm pump.
Honestly it is way to big and heavy of a hull for the 720 but my wife, probably like your cousin loves it and won't let me sell it. It is super stable, dry and doesn't use much fuel.
The prop Solas lists for that larger GTS/GTI is a Concord ST-CD-10/16. Let me know how it works as I might have to get one too if it is a big difference. The only thing that concerns me is they list the same prop for the same ski with a 85 HP 720 and the 110 HP 787 RFI so I have a little concern that is is not a one size fits all situation.