Kew Gardens is an LU station, part of the Turnham Green group.
Slightly off topic - what is a group in this context (do all lines have these station groups?)
"Group" was pre-Fit for the Future, there were 38 station group each under a Group Station Manager and each group had its own reserve of staff, they were replaced with 97 Areas some of which consist of just a single station like King's Cross/St Pancras under ...wait for it.. an Area Manager but most Areas share a reserve with other Areas in their "Cover Group".
Turnham Green Area consists of Stamford Brook, Turnham Green, Gunnersbury and Kew Gardens which with Hammersmith Area (Ravenscourt Park, Hammersmith & West Kensington) makes up District A Cover Group.
All lines had groups, when I joined LU I was a reserve Station Assistant at Edgware Road Group on the Circle Line which covered Edgware Road, Paddington and Bayswater. I then moved to Canary Wharf Group on the Jubilee Line which was Canary Wharf, North Greenwich, Canning Town, West Ham and Stratford but Canary Wharf was paired with Canada Water to form a separate group so the other four stations became Stratford Group. The five original stations are now Jubilee E Cover Group and split into three Areas.