Selling copies of blockchain database on disc
Is this legal under the terms of the GPL?

Bitcoin Core is distributed under the MIT license which explicitly allows this.

The content would essentially be the Bitcoin data directory (minus the wallet.dat of course), and, if possible, the client. Users could insert the CD, install the client, and have the entire blockchain as of the date the CD was published.

This is not recommended. Beyond modifications to the software (which can be caught with signatures) distributing a raw synchronized database is very vulnerable to tampering. The unspent outputs database can be silently modified to include non-existing outputs which the user would be likely to discover until it is too late. With knowledge of the people running a corrupted database an attacker can fork those users off the network very at their leisure, or if they accept zero confirmation transactions the victim can be directly defrauded at nearly no cost.
When a bootstrap.dat was distributed as a faster way for users to sync with the network it was always completely verified to avoid this type of attack.

Users could insert the CD, install the client, and have the entire blockchain as of the date the CD was published.

From a feasibility perspective this would actually be 60 CDs, or 7 DVDs at the time of writing.