What is the stride of a GAM interval

For interest, I was reading about the internal structures of a Microsoft SQL Server file. It's fairly obvious how IAM pages are connected. However, it is not clear to me where later GAM pages are to be found. It is written that the first GAM page is page 2 in each file. DBCC PAGE confirms this. The links above state there is another GAM page in "4GB" or after "64,000 extents." When I look in those places (they are not the same number) I do not find a GAM page. How many pages away from the first GAM page will I find the second GAM page (the "stride" of a GAM interval) in a SQL Server data file?