When you update firmware, you only need notice that:
If your current firmware is older than 2.2.2, you need update to 2.2.2 first. Then, you can update the latest firmware 2.4.6.
If you current firmware is newer than 2.2.2, you can update it to 2.4.6 directly.
You tell me that one of your gateway is 2.2.2. So, you can update it to 2.4.6 directly.