Hey Sunil
Sorry for the delayed response.
You could say "since you're not here" instead but Since in "since you've been gone" refers to the period of time you have been gone from when you left until now, since in "since you're not here" is usually used more like "because you're not here".
Does this make sense? I hope so, I'm sure an English teacher could explain this more simply for you if you ask them.
Rock On! - Grainger