Instead you can just swipe the spacebar left or right. It works much better than the iOS 3D touch cursor movement, which seems to false-negative on my force touch very frequently (at least half the time, for me).
Thanks for the hint on swiping the spacebar. I still miss the ability to move the cursor vertically and select with the trackpad but this at least makes Gboard usable for me.
I think I had the same false-negative problem you are describing with 3D touch trackpad, but it seemed to get a lot better for me with the iOS 9.3 release. Maybe I'm imagining it, but it seemed like there were improvements to whatever algorithm determines a user's intention for normal vs 3D touch.
On the other hand, maybe I trained myself to their algorithm. It seems like starting with light contact and increasing pressure is a more reliable trigger than just going in with firm pressure right away.
