We want to migrate SharePoint 2010 sites into SharePoint Online and we are going to use ShareGate Migration tool. But we are not sure if the tool (or other migration tools) do support automatically converting Classic Pages into Modern Pages? or we need to do this manually or using the https://docs.microsoft.com/en-us/sharepoint/dev/transform/modernize-userinterface-site-pages ? Thanks
It's very likely that you will need to use the PnP modernization tools to convert these pages.
I recommend you talk to ShareGate support team regarding any support for the converting of classic pages.