Hi @matan alter ,
Since SCOM 2019 only offers 64-bit agents, you'll need to install an older SCOM agent, for example the SCOM 2012 R2 agent.
You don't need a Gateway server to monitor older servers/clients, but if the server in question is located in a workgroup/DMZ environment, then you'll maybe want a Gateway server, so it depends where your server is located.
You can basically follow up on the link below, it should be more or less the same procedure:
https://www.scom2k7.com/monitoring-windows-server-2003-with-scom-2016
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Best regards,
Leon