On no account use glue, it would just squeeze back through the carpet and make an allmighty mess. Glue is only suitable for impervious backed carpets designed to be glued down. Anyway the carpet has stretched so you would still have excess material. Just needs stretching back onto the grippers and maybe trimmed. Always leave a couple of weeks before doing anything though.
Every carpet cleaner should have a relationship
with a decent carpet fitter. A good carpet fitter, there are some crap ones, will be able to fix any problems as regards stretching etc and not only that you will find you can pass the occasional job to and thro. In fact offer them a percentage and they may bring a lot of work.