I think this is because of the locale of the browser.
Change this line
google.load('visualization', '1', {packages:['gauge']});
to
google.load('visualization', '1', {packages:['gauge'],'language': 'en'});
Also, please refer to this page:
https://developers.google.com/chart/interactive/docs/reference#formatters
specifically the 'pattern' section of the 'NumberFormat' option.