Can a actiontec gateway wireless DSL router be

You could do it, but it's a bit of a pain in the ***** and probably not worth the hassle.
Actiontec DSL gateway was not designed to act as a "proper" router and therefore you won't be able to make it act like one permanently (well, unless there's a hacked firmware out there somewhere ;).
Of course, since it runs linux, you can telnet into it and manually set up the ip addresses on both wireless/wired interfaces and the necessary route between them. It will then work as a wireless access point for your comcast cable modem, but any changes will be lost every time you reboot the actiontec.
I've done it in the past with similar setup but quickly got tired of having to console the actiontec to add the ips and routes every time it crashed -and eventually got a proper wireless router.
So-it can be done, just not easily.
Have fun,
Raf