Hi @Rahamim Levi ,
You can do an in-place upgrade from Windows Server 2016 to Windows Server 2019, this is pretty straightforward, however Microsoft always recommends a clean install. If you ask me then performing one (1) in-place upgrade is "OK", performing more than one might higher the risk of corruption or unexpected behaviour.
For your Gateway servers I would definitely recommend a clean install, because this would involve too many in-place upgrades which may cause unexpected behaviour, and the Gateway server installation is quite a small thing to do in the end.
----------
If the reply was helpful please don't forget to upvote
and/or accept as answer
, thank you!
Best regards,
Leon