I did a google search and immediately found two ways to convert ExpressPCB files to Gerber. One requires you to pay $49 for a program and the other i s $5 for single sided or $10 for double sided Gerber files. Choose your po ison.
But you realize you have to deal with this crap each time you want to make a change to the layout? There are good layout packages that work as well i f not better than captive software. I've used FreePCB and it is capable an d easy to use. A lot of people use KiCAD which has a bit more learning cur ve, but can't be all that hard to use as many are doing it.