The trigger is slightly different on GBoard. Rather than long press and hold, you just touch the spacebar and drag right away. Press too long and the language options show up.
The other big difference with this is it's a 1-Dimensional traversal so it's like having the left and right arrow keys but not the up and down keys.
Looks like the UX is different on GBoard, it triggers language options when you have multiple languages set up, but works when you have only one language on. Thanks for letting me know!
It works with multiple languages too. Just tap and slide as if you're doing a horizontal scroll. Once in that mode, you can keep your finger held down and sliding as long as you want.