PGP compatibility

[R. Jackson]

Ok, this may make me sound stupid, but how compatible is GPG 1.0 with
PGP (any version)? I've read the FAQs and documentation, but I'm
confused about which versions of PGP will not work with feature X or Y
of GPG, and vice versa. For example:

Can PGP encrypted items using algorithms X and Y be properly decrypted
by GPG?
Which PGP versions can't read GPG signatures that use algorithm X?

I really want to use GPG, but I don't want to let incompatibilities that
I don't know about cause any problems for me.
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