While I still wait for my unlock code from AT&T and unable to check this myself, are you suggesting that SMS forwarding is handled by the Gear Manager as well, meaning you will receive text to you gear from the number on your main line?
Can you forward both calls AND SMS from that Tmo non-compatible Note4 (or other phone) to your Gear S? I know this has to be set up in the Gear Manager but wondering whether it is not a carrier specific feature...