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
CM12.1 and CM13 need a different partition size as gapps packages have become too big.
This can help:
https://forum.xda-developers.com/android/software-hacking/tool-lanchon-repit-data-sparing-t3358036
https://forum.xda-developers.com/galaxy-s2/development-derivatives/mod-increase-partition-size-t3011162