The G-55 padlock uses a 7x7 cylinder (a traditional 7 chamber dimple pin-tumbler lock), so yes it can be bumped. The only Mul-T-Lock model that definitely can't be bumped is the MT5+, which is not widely used yet. The "bumpability" of the other telescoping systems is shaky, but I've seen them bumped and have bumped my own before.
There's more information on all of the Mul-T-Lock telescoping systems available in the paper "Mul-T-Lock: Design & Security" at
http://www.lockpickingforensics.com/articles.phpdg