Short answer: originally the schoolroom of the Congregational Chapel in Union Road, Deal, and now part of the buildings of the Landmark Centre.
The first Congregational Chapel (known as the "Meeting House") was built in 1681. This building was pulled down and replaced by a new, larger, 500-seater chapel in 1803 which in turn was pulled down and replaced by the current building in 1882.
1The Sunday School was first built in 1810, then rebuilt in 1875. It was extended to its current form in 1909 as can just be made out on the foundation stone between the central window (GSV).
The 1892-1914 OS 25" map (this sheet revised 1897, published 1898) shows the Chapel and the rear door (to the vestry) which is the door on the left of the GTP photo. The small window to the right of the door shows similar characteristics to the main chapel, and quite different to the new schoolroom. Examination of the brickwork on this lower roofed section also indicates that this was the original vestry. The map shows three small premises where the schoolroom now stands.
The Congregational Chapel closed in 1974 and the building became redundant. It is now the Landmark Centre run by the Deal & Walmer Community Association. A new floor has been put in at gallery level to convert the main body of the chapel into two halls.
3 It provides "a community cafe, a community pub/bar, three Halls for hire (two with bars), a lounge with pool and darts (also with a bar), meeting rooms and training rooms".
2 The CA website continues: "The Centre, home to organisations such as Deal Radio and the Rotary Pirates, has rooms hired by a wide range of interest groups, societies, health and well-being focussed groups and is available for private hire. Weddings, parties, festivals, conferences, exhibitions, markets and other types of events are all welcome.".
2 One user of the centre is Christ Church Deal. This is an independent church founded in 1998.
4 After temporary locations elsewhere, the group now meets at the Centre.
3 I'm afraid I won't be able to put up the next GTP until tomorrow evening.
1https://familiesofdealandwalmer.co.uk/the-congregational-church/2https://www.dwchamber.co.uk/the-landmark-centre/3https://christchurchdeal.org.uk/about-us/landmark-centre/4https://christchurchdeal.org.uk/about-us/our-history/