Compress it first, the ciphertext PGP/GPG produces will resemble a stream of random bytes and won't compress much (if at all). PGP/GPG also internally does some compression (using zlib, I believe, so basically zip or gzip compression) but you might be able to compress it further by using 7zip and encrypting the resulting .7z file.

This way your wallet is compressed as small as possible and you have up to three layers of protection (wallet passphrase, 7zip passphrase and PGP key/passphrase) along with an optional PGP signature so you can prove the file wasn't tampered with.

Of course this is all overkill assuming you have a strong passphrase on your wallet.dat, but it's nice to know that if someone hacked your dropbox or gmail account it might take them a few billion years to get through the multiple layers of encryption and access your wallet.