There are m43 lenses form Panasonic and Oly that cover 100-300mm, Pany having the advantage of in lens image stabilisation.
The Oly 75-300mm 4/3 lens is also a good option but would need the m43 - 4/3 adaptor.
I also have the GF1 but mainly use legacy glass from 4/3 cameras(including the 73-300mm) I had before (I also have the Pany L1
Lots of options, but all down to money in the end.
Options in order of my preference would be:-
Oly 75-300 + adaptor