take it to a vet sap

if the tail gets infected it will travel up the spine and kill him

you can rinse the tail with salt water this will act as a disinfectant
but yeah if worried go to the vet for advice

its mostly not a problem as gerbils are extremely clean animals and will probably shed its tail if its a problem for it

but just keep an eye on it
if he becomes inactive or whatever then he most likely will have a infection
but if he is happy and active then he should be fine !!

hope that helps