Looking at the Gateway manual, the remote setup codes are those used by Universal Electronics. So some UEI remotes will work using either TV 1756 or 11756 (not a typo; I know the setup code used by the Gateway remote is 01756).

You can use a Comcast red button remote or some of the Atlas remotes used by other cable companies, or buy the RCRP05B ($20 at Best Buy, Walgreens, etc.)

Or, if the HomeSeer system can be programmed by inputting either IR protocol and device/function numbers, or Pronto Hex format, we can give you that information.
The IR protocol is NEC1, device 96.