What are the SharePoint default ASPX pages?
SharePoint 2003: site collection URL/default.aspx
SharePoint 2007: site collection URL/default.aspx
SharePoint 2010: site collection URL/SitePages/Home.aspx
SharePoint 2013: site collection URL/SitePages/Home.aspx
SharePoint 2016: site collection URL/SitePages/Home.aspx
SharePoint Online/Office 365: site collection URL/SitePages/Home.aspx
What does Sharegate preserve in a migration, and when?
Site Creation or Subsite Creation
Sharegate will set the homepage of the newly created site as an exact copy of the source site.
For example, if you copy a SharePoint 2007 site collection as a subsite for a SharePoint 2010 site collection, the destination subsite homepage will be named default.aspx.
In Sharegate, you can access your default homepage in Migration, Copy Content if the file is located under Site Pages (see default ASPX pages to know the location based on your SharePoint version).
When you copy content, Sharegate copies the content of the source homepage to the destination homepage. If it's from SharePoint 2003 or SharePoint 2007, it will be copied over to the destination, but it will not be merged into the original homepage. The setting for the site welcome page will also be adjusted to point toward the default.aspx page.
In this case, the two pages will coexist as follows, but only one will be the homepage:
site collection URL/default.aspx
site collection URL/SitePages/Home.aspx
The homepage is dictated by the setting under Site Settings -> Site Welcome page.
Can I set the homepage in SharePoint Designer?
Yes you can.
In SharePoint Designer, keep in mind the default.aspx is not in a library, while the home.aspx page is located in the library Site Pages (see default ASPX pages to know the location based on your SharePoint version).
You can set the preferred aspx page as your homepage before running your migration.