Formula 1 cars use semi-automatic gearboxes which must have eight forward gear ratios and reverse gear. Gears ratios must be made from steel.
Each team have to nominate gear ratios before the first race of the season. In first year of this rule (2014.) teams could change the ratios once, but in 2015. and 2016. the were fixed for the whole year.
For 2017 only, a competitor may re-nominate these ratios, which must be declared as a set, once within the Championship season. In such cases the original nomination will become immediately void, however, gearboxes currently in use and part way through their six race cycle, may continue to be used with the originally nominated gears until the end of that cycle.
Constantly Variable Transmission (CVT) is not allowed, just as traction control systems and devices that help driver to hold the clutch at specific point at the start of the race.
For safety reasons, all cars must have clutch that can be disengaged from outside of the cockpit. It’s usually placed just ahead of the cockpit opening and is marked by a red letter ‘N’ within a white circle.
FIA F1 2017. technical regulations can be downloaded here.