Are files encrypted with gpg compatible with pgp?

Yes, both GnuPG and the commercial PGP.com are implementations of the same OpenPGP standard; in fact, GnuPG was specifically designed to be PGP-compatible.

The only differences you might encounter are:

• Different supported algorithms. For example, GnuPG supports the Camellia encryption algorithm, while PGP.com does not.

  This is almost never a problem, though, as every PGP certificate ("PGP key") lists all algorithms that the owner's software supports (or more precisely those which the owner prefers to be used), so a message's sender can always pick a supported one.

• Various quirks and differences in really old versions of PGP (especially the original releases from before OpenPGP was standartized). However, both GnuPG and PGP.com can easily read (if not always write) messages in the old formats.

tl;dr: Yes, they are compatible.