BSD Slice Calculator
The GPART command for FreeBSD does the job, but isn’t that friendly. In particular, specifying partition sizes in logical blocks is confusing. You need to do some planning to figure out your partition layout, and then translate that into gpart commands.
The BSD Slice Calculator helps with this. Plug in the values in the blue cells, and it’ll give you the start and size of each partition in blocks, which you can then plug into gpart. Download it from here (ods spreadsheet).