The former Millwall and Cardiff City boss took over from Steve Evans for the final 18 games of the season with the Gills ten points adrift of safety.
They ended up relegated on goal difference with 40 points – finishing one place and two points better off than Doncaster Rovers.
A final day defeat to Rotherham United, which saw the Millers secure promotion to the Championship, consigned the Kent club to the drop.
"To have a tactics board on my first day would have been nice," Harris told Sky Sports afterwards.
"Minor details that people don’t see are major for football managers. The challenge was there, I accepted the challenge, I am emotionally invested in the challenge and that is why it hurts.
“I take responsibility for the football club - it has not been good enough. The whole football club has let its fans down this year. That will change when I am in charge.
"We’ve got seven players under contract,” he added. “All seven won’t be here next year.
"Players out of contract – some of them I’d like to keep, the rest can go.
“We’ve not been good enough. The balance of the squad, the fitness, the standards, it has been a disgrace and it has to change. It has been neglected to be honest."
Speaking about the challenge ahead, Harris later told the press: “I can’t make promises about next year but what I can control today is to say that the standards and professionalism at the football club will be much better than what I have inherited. That will be a given.
“Changes of personnel in the squad? We have a huge overhaul of players and there is always an element of risk to that because you try to recruit the players for the right reasons, it doesn’t mean you are going to get every one correct.
“There has to be learning curves in the football club, that if you don’t improve standards and you don’t drive forward [...]. I talk about taking steps. If they don’t take the steps, the football club, that I want to take then we are not going to be successful next year. That is the challenge to the football club. We have to be better.
“I am all in, as you can tell, I enjoy it here. I think I have galvanised the football club and brought the terraces closer to the pitch, the fans stuck with the players again (on Saturday) because, let’s be honest, we haven’t got as good players as the teams we were playing against.
“We can’t just slope from one week to the next as we have done as a club,” he added.
“The standards next season will be miles better. I will have players with more professionalism, I will have leadership in my changing room, I will have more quality and have a right good go in League Two.”