I don't have the G15, but if it has a Bulb mode and a spot to plug in an intervalmeter, then I'd be nearly certain you can use it to go longer than 15 seconds. I know on my old Rebel xTi, the maximum was 30 seconds, but when I switched to Bulb mode and plugged in an intervalmeter there was no limit-- only how long my batteries would last
. One way to check would be to switch to bulb mode and hold down the shutter button for 20 or 30 seconds. If that works just fine, then an intervalmeter will definitely work.
Also, it may be worth checking out CHDK (http://chdk.wikia.com/wiki/CHDK
) depending what features you're looking for. It's roughly comparable to a lot of the features that Magic Lantern puts in the DSLR's, only CHDK is for Point and Shoots.