ok, one question, in what episode Cal starts to call Gillian as Gillian and finally Gill?

Good سوال :D
The first time was in "Grievous Bodily Harm" when Terry asked Gillian for a drink ("Listen, Jill, آپ fancy meeting us for a drink later?") and then jealous Cal said: "That's Gillian, actually. And she's busy." Loved that scene :D

The first "Gill" was when Cal asked Gillian not to tell Emily about his 'little trip' to Afghanistan in "Secret Santa".