I would query that 772 dpi, probably a typo. I was looking at a Chinese publishing site just a couple of days ago. Nice description of offset printing but some of the language was "quaint"
CMYK in Gimp is a not wonderful. This is a video demo I made for someone else, It is Gimp 2.8 but same operation in Gimp 2.6
http://www.youtube.com/watch?v=2rby7r771D4&hd=1A good bet is to upgrade to Gimp 2.8 and consider the Partha version which AFAIK comes with the separate+ plugins.
If you use Gimp and export your image, then tiff, jpeg and psd are options. However separate+ will only import CMYK tiff. Not a bad idea to stick with that format.
Other options,
Much mentioned recently, Krita, open a RGB image, change to CMYK mode, save. (I assume this works in the windows version)
Command line ImageMagick will convert colour spaces and set a colour profile.
Not too sure of this one, but I think Scribus can export as CMYK. Not an application I use.
edit: a re-read of the OP
vinyl banners = big? probably the dpi will be 72 or maybe smaller if the banners are huge. 5' x 2' not huge so 72 dpi = 4320 pix x 1728 pix If the banner was to viewed at a distance of say 6' then 'rule-of-thumb' gives 95 dpi.
And another thought on 772 dpi, if not a typo then maybe it is the printing dpi which is not the same as the image dpi, example, for a photograph usually you use 300 ppi (pixels per inch) for the image but your inkjet printer will print at 600 or 1200 dpi
Which CMYK profile to use. Really, that is for the printing company to specify and sometimes to provide.