Apperloo,
Using a very long lens with a very small camera is tricky. It's not easy to hold it stable enough, and although OIS helps, long is still long. To be on the safe side, I would use shutter priority, minimum 1/500s with OIS and 1/1000s without. That means that you will often be shooting wide open and/or at relatively high ISO, but there really is no such thing as a free lunch. Remember that, at the longest end, that lens is 600mm eqv. Only ten years ago, that kind of focal length was more or less unheard of in the amateur world.
Correct technique is also important, and since the GF1 doesn't have a proper grip, you must have a relatively firm grip around the lens to keep it stable.
Hope this helps, at least a little bit.