If CM is already installed, gapps can be side-loaded by going into recovery, selecting adb, then on the pc using

adb sideload ./gapps-600-base-20151112-1-signed.zip

Gapps for CM13 can be found here:

http://www.devfiles.co/download/xXYn6WFz/gapps-600-base-20151112-1-signed.zip
More links are here:

http://www.cyanogenmods.org/forums/topic/download-cyanogenmod-13-cm13-marshmallow-gapps/

If storage on the system partition is limited, the pico version might work:

http://opengapps.org/?api=6.0&variant=pico