After exploring more, it's clear that there has been a standoff for months. NE didn't want to give SG more money until he showed he could play. SG wouldn't play until he got more money. There appears to be some debate in terms of how healthy Gilmore is. Some say he is fine, others are not as sure.
Since he was not on the opening day roster, his contract for this year was not guaranteed. So NE saves 14/18ths of his 2021 salary. It allowed them to bring in Jamie Collins and gives them a little cap room left over should they need it down the road.
Had NE started the year 3-1 instead of 1-3, they might have been more willing to keep trying to work things out with Gilmore. IMO, they are worse without him, but NE was not going to break the bank for a 31-year-old CB coming off an injury that still wants a big money contract.
It remains to be seen if Gilmore is healthy, if he is still the guy that was DPOY, and if he is happy and willing to give 100% effort on the field. I don't think the matter is settled, as it doesn't make sense he would play for CAR on the same low salary he refused to play for in NE. If the Panthers get the 2020 version of Gilmore, they will be disappointed.
This puts pressure on NE to extend JC Jackson, as he is also scheduled to become a FA next season.