Maemo Mapper does not use a Nokia-770-side "channel" like rfcomm - it communicates directly with the GPS device, which effectively isolates it from rfcomm-related bluetooth communication. rfcomm uses "channels" on the Nokia-770-side only in the sense of creating a file system device (which must have a unique name, hence an arbitrary "channel" like /dev/rfcomm0 or /dev/rfcomm1).

The "Channel Number" in Maemo Mapper's config refers to the channel listened to by the GPS receiver (similar to a TCP port number), which should almost always be 1. I don't know what maleffects this might have on other devices (like keyboards) that are listening on channel 1. If you want, you can try changing the channel number used by your GPS receiver or keyboard (I don't know how to do this - it's probably device-dependent) .