I guess that gThumb accesses the camera through PTP ("Picture Transfer Protocol"), and not as a USB mass storage device---this way you don't get the icon on the desktop nor a directory where the camera's memory card gets mounted, but the program communicates with the camera in a different fashion. If this is the case, you have roughly three options:
1) take the memory card out of the camera and use a card reader to access it as a regular mass storage device
2) if the camera allows you to, change (from an options menu of some sort) the connection type it uses from PTP to USB mass storage device; the names may be different in the camera, but there are two options if you can switch them---or then you can't, it depends on the model
3) try using a PTP file system
to access the files inside the memory card of the camera over the PTP. Like it says on the site, it's far from perfect, but might allow you to access the files without using a card reader..
My personal opinion is that you'd be easiest off with a card reader. But if you don't have one or dislike taking the card in and out all the time, see if the camera allows you to change the protocol; I'm not sure how Powershots deal with that, but hope for the best