Although designed to be robust and durable, some commonly replaced parts are:
Tubes
Replacement tubes are available for most makes of gun. It’s possible to use rainwater downpipe tubing cut to the right length but be sure to check the diameter and if it’s straightforward to fit nozzes onto the end.
Nozzles
A wide variety of nozzles are always useful, but because they are often cut to size for a specific job, they may not be ideal for the next job. You can also buy a replacement tube and nozzles together in one kit for most makes.
Plunger
If the head of your plunger becomes warped it will not effectively push your mortar material. This picture shows some examples of spare nozzles, a plunger end and a grout scoop.
You should be able to find everything you need by searching on-line or by contacting your supplier.