I have found some site use an ads pop-up to block their content, and force user to click the ads to make the block gone.
Is it allowed by GA rules?

What do you mean allowed by GA rules? Google AdWords rules? I would assume it is not.

Im refering GA for google adsense

I don't think it is advisable by GA. But make it confirm from experts.

Hmmnnn. That is a definite No No, certain to get your account banned
with lost of all earnings. If such sites are reported to Google, they will be gone immediately from the GA network.

It is not fair on the advertisers either. That is one form of click fraud.

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