I am quite confused by this error, it indicates that the proxy requires GSSAPI authentication but after reviewing the MOCKS source code I was unable to find any indication that it uses GSSAPI authentication.
In fact it would appear that MOCKS doesn't use any authentication at all.
I ran MOCKS on one of my test machines and was able to connect to it just fine with FlashFXP without any problems.
Did you make & compile MOCKS or did you run the pre-built binary? If you did perhaps something went wrong during the process?
Do you have MOCKS configured to connect to an upstream proxy?
Perhaps there is a problem with the MOCKS configuration?
Perhaps you might want to try another socks server such as
http://ss5.sourceforge.net/