You throw that into a text file, give it a .gpe extension, and make sure you save it with unix line endings if you're writing the text file on a windows machine (I've got no idea what programs let you do that - search this forum and you should find an answer). Then you run that .gpe instead of the normal one whenever you want to use the other resolution.
Oh, and I've just tested the width and height I posted earlier for widescreen output and it looks like it won't work (for some reason it doesn't scale like it should). Using a width of 312 and a height of 180 should do it though, and it's still close enough to 16:9.