The garbage collection routine always "rolls the garbage collection dice" when session_start() is called, except for when:
initialization errors(you configured php to use different session handler components, and they failed to initialize)
the current instance of the script executing already has an active session(you already called session_start in this page request).
gc routine happens after the session data is loaded. This means that even if the gc runs on this call to session_start, and the session is too old and really should be deleted, it's too late because it already loaded. It will probably get deleted(from disk, or from db, or memcached...whatever), but it doesn't matter because the data got loaded into memory already, and will get saved again at the end of the request. So yes, it probably becomes "alive" again depending on what session_save_handler you use(files handler would).
This is why if you don't want a very old session to get reloaded, you implement the logic in the script, by storing an age variable in the session.
The gc routine really doesn't care at all about what current session your app might be concerned with. It's non discriminating, and really is kinda a hack for what would generally be better implemented as a cron job. The files handler literally loops through every file in the directory, and if the filename starts with sess_, and the mtime is too old, it gets deleted. That's it.
Also, a note on the default files based handler. If you're on shared hosting, it's common for everyone on the server to use the same setting for session.save_dir. This means everyone can probably read/write your session files. It also means that, if someone on the server decides to do something like set the gc_probability real high, and set the gc_max_lifetime real low, it will affect your sessions too, because the gc routines that their scripts spawn will operate on the same directory where your files also reside. If you don't like that, change your session.save_path. But now you need to make sure you adjust gc_probability to a value suitable for your traffic level, because the dice wont be rolling as often if it's just you rolling the dice.