How much information can be gleaned about a gpg key by possessing both plaintext and ciphertext?

[Schlacta, Christ]

I know some encryption schemes reveal more information about the keys used
when an attacker has both the plaintext and the ciphertext.  In general,
how much information does GPG reveal in such situations?
How much plaintext/ciphertext matched data would an attacker need (An order
of magnitude is fine) before being able to reverse enough of the key to be
meaningful on fairly modern computers?
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