The Denmark Under-16 international, who is regarded as one of the best young talents in European football, has since agreed to join Dutch outfit Heerenveen.
"I don't mind the competition for places but I felt that Heerenveen were more interested in developing me as a person as opposed to Fulham where I felt like just another name in the bunch," Gartenmann told BT.
"The fact that the road to first-team football is shorter in Heerenveen had a lot of weight to it and their youth setup is also very competitive. So I'll be training with some of the best young talent on a daily basis."
A host of European clubs were also believed to be monitoring proceedings.