Cannot remove old gapps package after switching to omni rom nightly w/android 5.1

I have an asus tf300t. I managed to update from the final version of omni rom with android 4.4 to the latest nightly (8/27/2015) but the gapps package for android 4.4 is still flashed. I downloaded the package for android 5.1, wiped the system partition with twrp, and then reflashed the rom. (I did not wipe the data partition and internal storage.) After that I tried to flash the new gapps package but there was "insufficient storage in system partition." I rebooted the tablet and the old gapps package was still on it. How can I remove it without wiping the data partition and internal storage?