I bought a timing belt kit and it came with a water pump with a metal gasket. Do I need to buy some silicone sealant for that or can I just fit it as it is?
No, you don't need a sealant, unless it is explicitly mentioned in an eventual installation guide.
Some people still use sealant with those metal gaskets (with mixed results). In those cases it is essential to only add an thin film of sealant and wipe away the excessive amount.
The danger is that some add too much sealant. After mounting the pump, excessive sealant gets squeezed into the coolant circuit. Globs of sealant swirl around the coolant circuit, never dissolve (as designed) and soon or later block some important coolant channel.