Installing Gapps (space problem)
I guess you have two way. 


Go to Custom Recovery and wipe /System partition Then, reinstall Custom ROM. (Maybe /System partition have problem and after doing this step that's fixed. )   
Download Nano Variant of gapps and install on your device. It's very light. Go to Play Store and install other Google Application. (If you install Google Application from Google Play, they installed on /Data partition and /System partition be free.)