Install google APPs in Nexus7 with Cyanogenmod already installed
Its actually ok to install Gapps, or any other zip packages (if they're meant to be flashed), after Cyanogenmod has been installed. You'd use the Install zip from sdcard option while in Recovery mode (presumably ClockworkMod, if you followed CM's install guide) to flash the Gapps zip file. Then reboot your device and after that you'll notice that it will take a little longer to start up with a message saying "Android is upgrading". Wait for that to finish and then you'll see all of the Google apps installed.