Theoretically, the D90 will out perform the GF-1 if for no other reason that the D90 has a larger sensor. From the results of the tests in this link, you will see that the D90 also out performs the GF-1 in some important areas.
http://www.dxomark.com/index.php/eng/Image-Quality-Database/Compare-cameras/(appareil1)/294%7C0/(appareil2)/340%7C0/(onglet)/0/(brand)/Nikon/(brand2)/Panasonic
The other drawback at this time is The Nikon system has over 60 current lenses available it can use and the 4/3rds cameras have around 25 to 30 if you include Olympus, Panasonic and Leica 4/3rds lenes
http://www.dxomark.com/index.php/eng/Image-Quality-Database/Compare-cameras/(appareil1)/294%7C0/(appareil2)/340%7C0/(onglet)/0/(brand)/Nikon/(brand2)/Panasonic
The other drawback at this time is The Nikon system has over 60 current lenses available it can use and the 4/3rds cameras have around 25 to 30 if you include Olympus, Panasonic and Leica 4/3rds lenes