Ridership as of 1984 was 24 daily, to Fitchburg's 136. Fitchburg Line ridership three decades later in 2013 was roughly 2.7 times as high, which would put Gardner at about 64 daily. That, however, was with about one rush hour and one off-peak round trip. The heavy growth on the line has also been concentrated on the outer end of the line: in 2013, Littleton was at 7.1 times its 1984 ridership, Ayer at 4.9 times, Shirley at a whopping 11.7 times, North Leominster at 5.2 times, and Fitchburg at 3.8 times. Note this was before service increases to Littleton, North Leominster garage expansion (which will probably take some of the load off Shirley), and general speed and frequency increases across the line.
Will Wachusett actually reach 400 riders per day soon? That remains to be seen. But between overall outside-of-495 growth on the system, recent upgrades to the Fitchburg Line, and how consistently awful 2 is at rush hour, I would put the over-under for 400 daily riders at around 2020.
Even if some of those are poached from elsewhere on the system, that's still valuable. If someone driving from further west on 2 parks at Wachusett, they save (for the round trip) 12 vehicle miles traveled versus North Leominster, 50 versus South Acton, and 85 versus Alewife.