I don't think anyone has started a list yet. I haven't been to Rockport or that area in a long time and never for a garden tour. Open Gardens Days have been announced and I highly recommend visiting the one in Epsom NH which is part of Merrimack Valley Open Days July 14 and 15. The Monadnock area looks terrific - Sat June 16 and Sun June 17 with different gardens each day. And more to check out:

https://www.gardenconservancy.org/open-days/open-days-schedule

Rockport Garden Tour
Friday, June 29 & Saturday June 30, 2018
10am -4pm both days

Please add any tours you know about.