You post the map and the gametype together. Just put an extra paragraph in your post describing the gametype and than put the gametype link next to the map link. BTW I saw your signature and I agree with you on not playing H3 on september 25. Lower guns means better michinimas! Plus I just got a capture card so I might be making michinimas too!
Additionally, if you don't have a map for your gametype, try making a map (or handful of maps) that cater to it perfectly. Then release it is a map-pack revolving around the gametype. Perhaps try commissioning some skilled Forge artists to help you out? In any case, if I still had Live, I'd offer to help, but at the moment I'm at school in California, where the internet connection restricts the ports I need to access Live (that, and I wouldn't have the bandwidth for it anyway).
If you dont have a map and just want to post the map then I'd reccommend posting it in either Halo Forge Discussion or Halo discussion. With a vivid description explaining it and say what it's good for; what maps its suited for, etc.