If I understand correctly, your tires are 2 years old? How many kilometers do you think you've ridden on them? What does the tread look like?
How big is the gash? 1 cm long? 3 cm long? more than that?
Personally, I would see if it can be patched. It's so easy. Any old piece of duct tape or similar will do. . If you start getting serial flats, then it's time to change the tire. If the slice is more than 2 cm though I would rethink that and just replace the tire.