I am running proftpd on a gentoo box and can ftp to it fine over the command line. However, must GUI clients now try and switch to passive mode with PASV. Once this occurs, it tries to receive the directory listening for my home directory and it times out. I've been trying to find the problem.

I went to the logs... /var/logs/everything (is there a way to force proftpd to log to its own file?) and first found problems with proftpd trying to set up USEIPv6_V6ONLY: protocol not available. So I set UseIPv6 off in /etc/proftpd/proftpd.conf. Connecting again caused the logs to have entries regarding passive and PassivePorts and MasqueradingPorts, along with timeout entries. So, I added PassivePorts 49152 65534 as noted from an article I found.

I still get:

The machine is on the DMZ of my router, so should have no firewall rules to contend with.

\\EDIT To correct poor formating the best I could.

It's either an IP adress problem and/or a port blocked by a firewall.
From a gui, try to enable debug output.
At least from command line, there is debug output:
ftp server
<user,pass>
passive

It will show something like:
227 Entering Passive Mode (127,0,0,1,9,128).
Be sure that the IP adress is accessible from outside: if the server tells the client to connect to your internal adress then it won't work.
Now try ls
If it doesn't work, its a firewall problem.
From the client, try to see if it can connect to 127.0.0.1:2432 (2432=9*256+128, so adapt the IP and port)
You can use netcat for example
nc -vv IP PORT (nc 127.0.0.1 2432 in my case)