When installing GSI ROMs or installing custom ROMs, you may run into a problem where you run out of storage in the system partition when installing Gapps or any extra zip files. Well, TWRP actually can resize the system partition for you(without loss of data) so you can resize your system partition and install more zip files such as Gapps.
Now, some TWRP versions do not show you this option under the “wipe” menu. You can also make a backup of your boot, system, and data partitions then wipe the system partition(which will automatically resize it) then restore your backup. That will work for any TWRP that does not give you the resize option.