Gigabit is the slowest reasonable fiber speed. Even 10-gigabit is now very cheap to the point that some gigabit equipment is just 10-gigabit equipment operating at a low speed. They could artificially throttle you to even less, if it made business sense.

It's not a committed rate. Your individual line is a gigabit, but the upstream from your whole block is 10 gigabit so you can't all use it at once. Your guaranteed rate is probably have more like 20-50 Mbps, if that's what's confusing you. But it's extremely rare that everyone tries to use their gigabit all at once.

If it's a Passive Optical Network, you might be sharing a gigabit download with your block - you all share the same fiber - and you get substantially less than a gigabit upload due to the need for timeslotting. Gigabit PON is obsolete though, now you'd get at least 10G PON.