Kindly check below details for GAMLN Tool  (Lotus Notes Migration)
System Requirements for Migration Server
  • Lotus Domino Server 6.5 or higher for Windows (32 or 64 bit).
  • If you are adding dedicated migration servers to your network, those servers must be registered as part of your existing Domino Organization.
  • Microsoft Windows 2003 Server or higher. 2 GB RAM (minimum)
  • Administrator access to mail files. We recommend that you use a generic Google Apps
  • Account with administrator privileges for your domain for the purposes of the migration and not a real user account.
  • Microsoft Core XML Services 6.0. If your version of Windows does not include Core XML Services 6.09, you can download it at:

Microsoft Core XML Services 6.0 is included with various Microsoft products (for example,
Microsoft SQL Server and Microsoft Office). To see whether you have Microsoft Core XML
Services 6.0 installed, you can search your Windows folder for msxml6.dll.

Note: GAMLN can be co-located with your Domino mail server if that computer meets all of the GAMLN system requirements, but this is advisable only if your users have stopped using Lotus Notes mail prior to migration, because GAMLN consumes significant server resources as it migrates.

Reference link

To start the migration for lotus notes,
1) Download the the GAMLN guide from below Link

2) Refer Below Video for step by step guide for migration process.

You wish to perform the migration first, and then change the MX records. This would not be the best because once the migration is over, e-mails received by Dominos will not be sent to Google Apps. So you would lose those.