The EFI partition is a required partition used by the UEFI partition schemes.

GuixSD does not require anything special compared to other distributions.
I recommend you have a look at

	https://wiki.archlinux.org/index.php/Partitioning
	
for general guidelines.  Then you choose how to partition your drives as you
wish.  Using the UEFI partition scheme, you can have two partitions:

- EFI
- root

You are free to add as many partitions as you want.
"Free space" is wasted space, make sure that your partition scheme covers the
whole drive.
-- 
Pierre Neidhardt