We have performed multiple migrations using the Microsoft cloud migration tool, and it works quite well. The tool creates a new D365 Business Central company and migrates all GL historical data. It also brings in the Chart of Accounts, Vendors, Customers, Items, etc. Here is the Microsoft documentation for the tool: Migrate from Dynamics GP to Business Central online - Business Central | Microsoft Docs. I also did a session at Summit 2021 on this topic. If you are interested in a recording of the session, I can send it to you.
If you are only looking to bring in GL data into BC, the tool would not be appropriate. You can, however, bring the data in manually using a configuration package. Because you need to bring the data in as an unposted transaction and post it, I recommend that you bring in summary GL data by period rather than all historical transactions.
I hope this gives helps.
