i want to make GSA to prioritize (in the results) content with some specific metadata . Is that possible? Thanks!
Yes, it's possible. Metadata is one of the options for biasing results.
The admin console help tells you how to do it: 7.4 Admin Console Biasing Help
It's worth pointing out that biasing is more art than science. You may need to experiment a bit.
You should also make sure that you enable Advanced Search Reporting (ASR) so that the self learning scorer is enabled. This is where the GSA adapts to user behavior, learning to deliver better results over time. Links that get more clicks automatically move up in the search ranking.