Gilmour was speaking as part of the BBC documentary Series A Scots: Return to Naples, which revisits the Scottish contingent at the club following last season's Serie A title win.
The former Brighton midfielder reflected at length on the night Napoli beat Cagliari to clinch their fourth Scudetto, on life alongside McTominay in Naples, and on Scotland's play-off victory that secured their place in the World Cup.
On the title-winning evening, Gilmour described a city already in celebration before a ball had been kicked.
"The fans were celebrating before the game, we could see that on the way to the stadium. I think when we went out on the pitch there was one objective — go and get the job done. Scott scored in that game and I think that relaxed things a bit. And then as soon as the final whistle went I was absolutely delighted. I think it was also a bit of a relief, because you work hard all season to win the trophy and in that moment I felt a bit of everything."
On his relationship with McTominay, Gilmour was candid about a player he sees as far more influential than he gives himself credit for.
"He's a very calm guy, he likes to relax. He trains and then goes home. I'll invite him to dinner or to play cards or maybe watch TV. He's a very simple person, he loves the quiet life and doesn't give himself enough credit for what he's done. Absolutely not. And as a friend I can say that. What he did in terms of goals at Napoli last season, and also with Scotland in difficult moments — he pulled the team out and got us to a win or a decisive goal. For the team, for the fans, for the kids who watch him and idolise him, he is a point of reference."
The most dramatic moment of Scotland's qualification campaign came in the play-off, when McTominay scored a stunning bicycle kick against Denmark to seal the country's first World Cup appearance since 1998.
"Scott scores and tells me, 'Bill, you won't believe it, I've just scored an incredible goal.' That game was a battle. I hate to say it but as soon as it happened I thought — I can't believe it. The goals were something special. And the stadium — Scotland is like that, so much passion. It was a special evening. Scott scored the best goal."
Gilmour was bullish about what Scotland can achieve this summer.
"There's a bit of the unknown factor, but we're going there and we want to do well. The objective is to win the first game and then take it step by step. But we'll go there with confidence and show the world how strong Scotland is."
McTominay, who joined Napoli from Manchester United for €30.5 million in the summer of 2024, scored 23 goals and contributed 10 assists in his first season and a half at the club. Napoli are understood to be working on extending his contract, which currently runs until 2028, by a further two years.