i would not go for g2f for map editing and converting... why? g2f was implemented for static pictures... and when g2f is optimising the fonts/chars used in a pic (so its not always 1 charset each 3 scanlines...

of course sometimes even just 1...)
now...when you are turning your bitmaps of your level data into a charset... its more likely that each of your level spices get another order of the charset or use different amount of charsets in total... so i guess horizontal scrolling will be difficult as your display gets corrupted and dont forget that how do you switch "fonts" on the fly when scrolling 2 different bitmaps?
i would go for a standard map/tile editor which use 1 font for the whole level where you can build up your map.
some use envision... some (like me...) use charpad from c64...