TAHT (Tahiti Time) is 10 hours behind GMT/UTC. When it's noon in Pape'ete, it's 10pm in London, 2pm or 3pm in Los Angeles and 9am (the next day) in Sydney; the region is just two hours east of the International Date Line.
There is no daylight-saving time.
The Marquesas are a half-hour ahead of the rest of French Polynesia (noon on Tahiti is 12.30pm in the Marquesas).
The Gambier Archipelago is one hour ahead of the rest of French Polynesia (noon on Tahiti is 1pm in the Gambier).