What does "Gitti gideli" mean? How can I translate it?

And what does Kontör mean?

 You can translate "since s/he has gone/left" and "kontör" meaning is "credit/top up for cell phones"

 Gitti gideli means " Since he/she has gone"

                         " Since he/she has left "

 Gittim gideli = Since (since the day ) I have left (gone)

 Gittin gideli = Since  you have left (gone)

 Gittik gideli = Since we have left (gone)

 Onlar gitti gideli = Since they have left (gone)

 For example= O kursa gitti gideli ingilizcesi ilerledi= Since he/she attended the course his english has improved.

This usage of repeating verbs is seen quite often in a daily life.

Kontör means mobile phone credits.

For example = Kontörüm bitmek üzere ( My mobile phone credits almost running out )

 Tunci, I like the way you added "the day" to this. It makes it clearer

Since has 2 meanings in English. One is "because". The other is "from that time onwards".

Gitti gideli isn´t the first, it is the second one: since the day you left.

(the first would be gittiği için, or gittiğinden dolayı in Turkish).