Comparing Chiang Mai to Koh Samuri is like comparing chalk to cheese. Chaing Mai is the 3rd largest city in Thailand with all the facilities you would expect whilst Koh Samuri by comparison is a small island with little to offer accept a coast line. However the smoky season in Chiang Mai gets worse each year and and is enough for many, including myself, to relocate to places such as Hua Hin
Generally speaking a gite is a small cottage or outbuildings that have been converted to provide short term holiday accommodation though people do live in them long term.
As mentioned you can not enter Thailand on a retirement visa if you intend to work
This is not a requirement if you apply for a O visa based on retirement. Just come on a 60 day tourist visa, extend that and apply for the o visa when you are here. No medical or insurance required