Although the name is the same Giant Gecko, please think of it as a different family.
There are several families of EFM32, but I called the new generation product group released relatively recently as Series 1, and the product group before that as Series 0. The "S1" in Giant Gecko S1 stands for Series 1.

The families belonging to each series are as follows. The number in () is the ARM core used.

Series 0:
Zero Gecko (M0+), Happy Gecko (M0+), Tiny Gecko (M3), Gecko (M3), Leopard Gecko (M3), Giant Gecko (M3), Wonder Gecko (M4F)

Series 1:
Jade Gecko (M3), Pearl Gecko (M4F), Giant Gecko S1 (M4F)
 
Below is a comparison chart between the Giant Gecko and the Giant Gecko S1. The Giant Gecko S1 is equipped with 2MB of flash memory, making it possible to implement even larger applications. In addition, peripherals such as interfaces such as CAN and Ethernet that are installed for the first time in the EFM32 family, encryption accelerators and random number generators installed from Pearl Gecko are also abundant.