I have a Panasonic G1 and also the 45-200mm lens.It's a good enough lens,but as with all zoom lenses at full magnification,you really need a tripod to keep it steady.
You can manage(just)with the mega-ois switched on,by taking your time and steadying yourself.
I don't know about the G10,but mine has an upto 4x digital zoom hidden away in the menu,which you definitely need a tripod/monopod!
There are adapter rings available should you wish to buy other lenses,but not sure how much these are.
HTH.