That's really personal. I think 275's are too wide in front, but others would not agree. If I were you, you might want to consider 255's in front and 275's in back. However, the 275's may not fit on your rims. The stock rims on the GTs are actually 7.5 inches wide and not 8 as most people think. Do you have after-market rims?
Do you loose traction with wider tires, of course not...on dry pavement. Wider tires allow for more rubber contact which generally increases traction. However, wider tires hydro-plane easier and suck in the snow. A narrower tire is always better for the snow.