Rahul is right, but I might contribute to this (I also got the i9100 and ran into the same problem). I had the problem that my google apps were already bricked so i could not install any helper apps.
TAKE CARE: This may brick your system if you are not sure what you are doing.
When upgrading CM12.X to 13 you have to install a new gapps version. The problem is, that with CM13 the old recovery system (which you need to install gapps) is being replacedby a new one which lacks the busybox. My solution so far: flash back the old recovery system using Odin (I used clockworkmod as recovery system which has busybox). Download the gapps and latest cyanogenmod on your sd card (zip files) and insert it in your phone. Boot into the recovery. Clean and wipe your system. Install cyanogenmod from zip file and directly afterwards the gapps zip file. This might take some time. Then reboot your system.
CRITICAL NOTES: - When installing cyanogenmod from sd card, your recovery will again be overwritten with the new embedded recovery system cyanogenmod (that one that is lacking the busybox). - If you haven't resized your partitioning, installation of gapps (I used the smallest version called pico) will fail because of insufficient storage. If this is the case, you also have to flash new pit files and format sdcard0 (vfat), system, data, preload and emmc (vfat). Then you may install your custom rom and gapps from recovery.
Links: Pit files different sizings: forum.xda-developers.com/showpost.php?p=60473209&postcount=1106 (pick one with 512MB preload (!) just to be sure) CWM Recovery: http://galaxys2root.com/cwmtwrp-recovery/ Philz Recovery for flashing pit files and formatting filesystems: http://d-h.st/SvM
Unfortunately this procedure have to be repeated once you have to upgrade your gapps. I found no better solution so far...