Goals from Adam Rundle, in the 24th minute, and Chris O'Grady five minutes from time, saw the Mariners crash to a fourth consecutive defeat and slip into the League Two relegation zone.
"There are lots of things here that we're going to change. I've just said to them in there I can take all the flack and the criticism, some of it's rubbish and some of it's not," said Newell.
"I've seen some signs in the game that there are a few people who want to roll their sleeves up and want to compete. What we have to do is dig out the ones who don't want to be here and don't have the stomach for the fight. I'll make sure that they are with me now otherwise they can go.
"I don't know if the end of the season will be long enough to keep us in the league. I have not got a crystal ball. Judge me then.
"I've had the backing of the chairman, he is standing alongside me and that takes guts, so we'll have to suffer it for however long it takes. You don't just get this club away from where they are overnight and I think everybody is realising that now."