how to setup multiple ethernet card into one gateway

Rather than mind reading, could you provide the details of the node, the gateway you're attempting to work with, and how you will have these ethernet cards physically added to your network?

Generally, Proxmox will have at least one IP after installation (The node displays this after booting, to let you know where the GUI is) which will allow you to talk to the proxmox node. After that, it is a matter of setting the configuration for each individual Ethernet port. However, this does beg the question, have you read the documentation concerning configuring networking in Proxmox? (https://pve.proxmox.com/pve-docs/chapter-sysadmin.html#sysadmin_network_configuration)
I would also recommend you familiarise yourself with the GUI: https://pve.proxmox.com/pve-docs/chapter-pve-gui.html

You may get a better response from the network configuration forum over here as well: https://forum.proxmox.com/forums/proxmox-ve-networking-and-firewall.17/

Right now there's very little I can say on how to help you, once there is more info, then there will be some direction :)