Some cordless drill drivers have plastic gears and others have metal ones. Both types have positives and negatives.
Plastic gears
Plastic gears are usually found on less expensive or lower voltage drill drivers that are designed for light duty DIY work only (e.g. driving smaller screws into softwoods).
Compared to metal gears, plastic ones tend to be weaker and may break or become worn more easily if subjected to heavy loads (i.e. if the drill driver is used to drive larger screws into tough materials e.g. metals or ceramics).
However, compared to metal gears, fixing or replacing plastic gears is easier and less expensive.
Metal gears
Metal gears are usually found on semi-professional, higher-voltage drill drivers that are able to drive small and large screws into a variety of materials (e.g. hard and softwoods, plastic, metal etc.)
Compared to plastic gears, metal ones can withstand the pressure of working with tough materials. A metal gearbox is therefore less prone to wear and breakage compared to a plastic one.
However, compared to plastic gears, metal gears require a more expensive and extensive fix if they become damaged or broken.
Please note: If you do require the gears on your cordless drill driver repaired or replaced, you should contact the manufacturer.