Both players arrived on deadline day last month, with McTominay signed from Manchester United and Gilmour from Brighton.
Manna told Radio Crc: "Scott McTominay really wanted to come to Napoli and this was satisfying and should be for all of us.
"If a Manchester United player chooses to come to Naples, despite having other opportunities, we should be proud. It's an operation that started a long time ago, already in the first days of June I had had some contact with the agent, but I didn't think we would take him.
"We continued to maintain these relationships, until the moment we sat down with Manchester United and we understood that it could be done, also strong in the will of the player."
On Gilmour, Manna also said: "Billy is the first midfielder we have actually negotiated and defined. We had practically done it in July, then there are market dynamics that slow down these transfers. We have always remained in contact with the player and the club, then when the situation seemed to have unblocked, there was (Matt) O'Riley's injury.
"They asked us to make the agreement for January. We had a stalemate, but then the player strongly wanted to come to Napoli and correctly asserted his will with the owners and the coach. So on the last day we had the opportunity to make this agreement. We had everything ready and in half a day we managed to do everything."