Gaia Data Release 3. The Gaia Andromeda Photometric Survey

Context. As part of Gaia Data Release 3 (Gaia DR3), epoch photometry has been released for 1.2 million sources centred on M 31. This is a taster for Gaia Data Release 4 where all the epoch photometry will be released. Aims: In this paper, the content of the Gaia Andromeda Photometric Survey (GAPS) is described, including statistics to assess the quality of the data. Known issues with the photometry are also outlined. Methods: Methods are given to improve interpretation of the photometry, in particular, a method for error renormalisation. Also, use of correlations between the three photometric passbands allows clearer identification of variables, and is not affected by false detections caused by systematic effects. Results: GAPS presents a unique opportunity to look at Gaia epoch photometry that has not been preselected based on variability. This allows investigations to be carried out that can be applied to the rest of the sky using the mean source results. Additionally, scientific studies of variability can be carried out on M 31 and the Milky Way in general.