A late goal from Simeon Jackson gave the Gills a 1-0 win over Shrewsbury Town in the League Two play-off final at Wembley on Saturday.
The Kent side become the fourth side promoted to the third tier of English football - following in the footsteps of Brentford, Exeter City and Wycombe Wanderers.
But Stimson is aware he needs to add more quality to his side if they are to survive in League One next season.
"We need to improve the squad and we have to try and become a good League One side," he said.
"We had already arranged meetings with some players for next week regardless of what league we were going to be in.
"It's massive financially. There are several local teams we can attract and with Leeds, Norwich and Southampton coming down we will have some big gates.
"We brought over 30,000 fans to Wembley today and we need to attract as many of them as we can every week."