The Bantams crashed to a 4-0 defeat in Kent and remain bottom of League One.
Hopkin was so furious with City’s players that declined to speak to the media after the game.
In a club statement, he said: “Saturday was bitterly disappointing for everyone connected to Bradford City and there are simply no words to describe the way in which we collapsed in the second half.
“It was unforgivable and wholly unacceptable. I want to take this opportunity to apologise to the supporters who made the long journey to Gillingham and spent hard-earned money to endure what was served up.
“I would also like to apologise to the members of the media who I have built a good working relationship with over the past eight weeks, for not speaking with them after the game.
“David Ball’s first-half goal should have stood and I think everyone would agree in saying we would have been good value for a two or even three-goal lead at half-time.
“Half-time came at the wrong time for us and the right time for Gillingham. Their first goal was against the run of play and we simply didn’t recover from it.
“The attitude of the players left a lot to be desired and we blew a massive chance to earn a vital three points, in what was a significant fixture for the club given our current predicament.”