I am unable to set Gear to (none). I can choose different gear, but not the none option.
When I click OK the dialog closes immediately, rather than briefly showing a spinner like it does for other gear changes. When I re-open it, gear is set back to its previous value. No errors in browser console.
Seen on this activity: i101580664
Never used it for gear but if null value is not accepted, usualy a space works.
Not really relevant, as the gear selector is a drop-down, not a text input.
None is the default. Did you add something and now you want to delete it?
It works from the activity list view when using Edit - Edit. Gear (remove).
So seems like a bug making it not work on activity dialog.
Bumping this topic, as I am now experiencing it as well.
As MedTechCD says, Edit - Edit. Gear (remove) works for a list selection, but the single activity dialog does not (from list or from activity).
i also can’t set gear to none.
i meanwhile added shoes called Feet that i bought on my birthdate. unfortunately i am not able to calculate kilometers so far. 
The activity ID is i140301668
There’s a default gear assigned, but I want to change it to (none) because this is just a temporary bike I used.
No matter how many times I tried, it is not effective.
I can, however, change it to another bike that I added.
As a workaround, see this:
For a similar situation I’ve created a “Random Granpa Bike” and “Random Old Shoes” that I set when not using my regular stuff.
So all temporary gear is collected by a “Random Something”
I have gear filters set up to assign bikes to my activities automatically. I just got back from a cycle tour in France where I was riding a rental bike. I attempt to select (none) for the gear used in each of my activities of the tour and click “OK”, but it immediately reverts back to the gear determined by the filter. It appears I cannot save the activity without any gear when it identifies gear based on a filter.
A couple of post higher up