Is it already available? If not, any plans to include this information?
No, it is not available. And it is hard to tell whether Garmin has any plans to include it, but I rather doubt it. You'd need to ask them directly, your request is not very likely to get their attention here, on the user forum.
As a workaround, I'd recommend putting the current gear mileage into the notes, when you complete the activity. If you want to have the value searchable, or displayable on the activities list chart, you could consider using one of the fields that you do not bother about (for example the Elevation Loss, or the Max Cadence), instead (or in plus) of the notes.
Thanks for your answer! I may adopt the "notes" solution for some activities like a competition. But it would be nice to just have it available as a feature so I don't need to be so farsighted for regular activities.
How do you suggest to "ask them directly"? Mailing Garmin support?
However, they rarely react at all, unless they have thousands of identical requests, so I would not hold my breath, waiting for it to be implemented any soon.
