Correct me if I am wrong about くれる form. It means any action which we received from someone.
How does it change the meaning if じゃないの is added?
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First, I would like to make sure that the sentence you wanted to ask about is actually:
as you wrote it, and not:
because those two sentences mean completely different things.
「言ってくれるじゃないの。」 is much more nuanced/complex than the words used might suggest. I will talk about its grammar first, then its meaning and nuance.
「じゃない」 here is affirmative. It is not the negative 「じゃない」.
It is affirmative because the negative form of 「くれる」 will always be 「くれない」 and it will never be 「くれるじゃない」. If someone said upon seeing my new shirt 「かっこいいじゃない！」, that would mean "That (shirt) looks great!" The negative form of 「かっこいい」 is 「かっこよくない」 and never 「かっこいいじゃない」.
Sorry for the boring explanation, but I had to explain it because I have encountered quite a few Japanese-learners who did not know anything about the affirmative 「じゃない」. "Quite a few" was actually an understatement.
Back to the sentence in question..
「言ってくれるじゃないの。」, therefore, means practically the same thing as 「言ってくれるね。」.
「言ってくれる」 here is nuanced as I stated above. It is not the same as the "regular" 「言ってくれる」 that means someone tells you something (for your benefit).
Here, 「言ってくれる」 means that someone has the guts to tell you something negative that others would probably hesitate to tell you.
Thus, a sentence like 「言ってくれるじゃないの。」 is usually uttered with a degree of sarcasm in the speaker's tone.
"Wow, you've got the guts to tell me that. Thanks!"
is the nuance of the sentence in question.