If OpenDNS has been configured to put a block on using Google.com, then you run into a massive problem as you will not be able to resolve any Google address including mail.google.com.
Any access to Gmail will then either have to be extranet through a device you control that can get access outside the protected network, or to have gmail forward everything to a service you can access internally. It's one reason smart-phones have gained popularity.
Using another client like Thunderbird will not take care of the problem if pop.gmail.com and smtp.gmail.com are blocked. You still have to access the Google mail servers and the block is on resolving Google addresses through DNS.
I think Chris was thinking about another way of doing it. If the DNS doesn't allow resolving the name, then directly use the IP address in the client (Thunderbird, et al.) I've never been forced into doing that, but Thunderbird's free to experiment with.
The only problem with it is that Google often uses multiple addresses that they round-robin so you might have to fiddle with changing the IP addresses every so often. Currently, OpenDNS is reporting the following for pop.gmail.com and smtp.gmail.com, you'll have to do it outside the block to see what your area returns: