As exclusively revealed by the News of the World on Sunday, the Boro boss was fighting for his job despite his side mounting a challenge for promotion back to the Premier League.
The Teessiders defeated Derby 2-0 on Tuesday night but Gibson is worried about a drop in attendances at The Riverside and made the difficult decision to part company with the former England international.
"When I sat down and considered our start to the season, I felt that the league table was actually more favourable than some of our performances," revealed Gibson.
"Then I looked at the important games against the teams immediately around us, and took those results into account. In general, the results and the performances have not been to the level I would have hoped for.
"Before last night, our five home games had brought in seven points. That will not get us promoted - and promotion is the only objective for the season."
Gordon Strachan has been made the bookies' favourite to take over the post while the North East club have also confirmed that former Crystal Palace manager Alan Smith has left his position as football consultant.
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