There are three partition types: primary, extended and logical, each one with their own pros and cons. Basically, you can create up to four primary partitions (if you're using an MBR partition scheme, but let's not focus on that), one being an extended one (some operating systems don't recognize more than one), and inside that extended one, multiple logical ones. You cannot boot up an operating system inside a logical partition, and that's why we put the /boot partition (which hosts grub) inside a primary one (wow, not it all makes sense!).
As you can see, your extended partition is already fully allocated in three logical partitions, and that's why gParted and Disks only give you the option to create a primary partition.
It is reasonably safe to create another primary partition. It won't break your system's boot.