Gilmore was a huge shock because, as I noted earlier in this thread, it was a very un-Belichickian signing. Because the Patriots rarely pay people.
Cooks wasn't signed - he was traded for and is still on his rookie contract.
Also, the Patriots defense is 5th-best in the NFL in points allowed, and best since week 5, so I'm not sure where you get that it's been awful this year.
If you look at OverTheCap and sort all players by avg $$/year, the Patriots' first player is Brady, #15, then you get Gilmore at #59, Solder at #111 and McCourty at #124. They don't pay guys bank-breaking contracts. They just don't.