I have a nexus 320 pump fed automatic. I want to change to gravity fed. What do I need to purchase to do this.

Nexus Filters are supplied ready for pump fed installations. To convert it to gravity fed, please refer to the manual. https://evolutionaqua.com/pub/media/bss/productattachment/Nexus320_Manual-A5_Size-March-2021.pdf

You would also need to purchase spares for the automatic version. We have a spare parts list online to help you identify which spare parts you would need with the correct ea product code. You can purchase these online or with one of our dealers. 
https://evolutionaqua.com/pub/media/bss/productattachment/Nexus_Spares_List_-_Updated_1.pdf