Student or Learner
What does this sentence mean?
- It' s not a given, it's a gift.
As your question is about It's not a given, it's a gift, that would have been a better thread title.
"A given" is something that is sure, certain, taken for granted.
Perhaps the meaning is "Don't take this for granted. You can't assume you would have it. It was a gift, not an obligation, and you shouldn't have had any expectation about receiving it."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.