Out of the bottom four at the start of the day, the Magpies were on course to stay up after Graham Burke’s 61st-minute goal.
But John Egan’s header two minutes from time dropped County back into danger before Brennan Dickenson and Luke Norris’ stoppage-time goals rounded off a miserable few minutes for Moniz’ men.
“It’s a disaster. I can’t describe it. There were five minutes to go, we had it in our own hands, and we gave it away,” Moniz said.
“The first (Gillingham) goal starts with a foul on (Mustapha) Dumbuya, and to give that moment away was crucial. It’s a 100 per cent foul for us. But of course we must mark better – we conceded too easily.
“After the last game, with today’s away attendance and the score with five minutes to go, the players did not deserve this. It’s a massive shock at the moment.
“In bad times, character will show. It’s like a funeral in the dressing room. Everybody was so hungry to take the three points, and when it slips out of your hands in the last five minutes, it’s something you’ll never forget.
“At the moment, words about next year are unnecessary. We will give players feedback on Monday or Tuesday, and then decide who wants to stay and who wants to go.”
While the Magpies end the season two points short of safety, the Gills finish 12th – their highest position since promotion to League One two seasons ago.
And Justin Edinburgh, appointed as manager in February, admits there is plenty more to come from his side.
“I’m delighted. People could have said there was nothing left for us in this game, but this group of players have the self-motivation and the winning mentality.
To come from behind to win, it’s a fitting tribute to end season,” he said.
“Their goal was disappointing, then John pops up with an equaliser and then we pushed on from there.
“Within the dressing room, I think the goalscorers meant a lot to the group. Luke hasn’t scored here before today, and it doesn’t matter how mentally strong you are, that affects you. For him to leave here having broken his duck, that will stand him and the club in good stead.
“We beat last year’s points tally and we finished higher. In my time here, the win ratio isn’t far off 50 per cent.
“But there’s room for improvement – we missed the play-offs by seven or so points and I think we could have accumulated them.”