GPL License enforcement

Hello,

I have a question about the GPL which has been bugging me for some
time now, and some enlightenment would be appreciated.

Suppose I obtain a copy of a GPL'ed internet chess server and modify 
it to suit my needs. I put this modified version on production.

Do i have to distribute the source code with my modifications to
anyone who asks for it? Or can i, sort of speak, close my branching
of the code?

TIA

Jose