The acronym Gaia is going to be changed in Simbad into Gaia DR1.
The acronym Gaia NNNNNNNNNNNNNNNNNNN is going to be changed in Simbad into Gaia DR1 NNNNNNNNNNNNNNNNNNN. The number is based on the HEALPix tessellation sky pixel indexing. For some stars this number will change between Gaia DR1 and DR2; the same number could even correspond to two different stars. For a few months Simbad will automatically interpret "Gaia" as "Gaia DR1". But shortly before the release of Gaia DR2 the synonym will be canceled, so that users will have to specify the version number in their requests.