Other forms: gashed; gashes; gashing
A gash is a deep cut, like a gash on your knee from a biking accident, or a gash in the earth caused by workers who are digging up a broken sewer.
The noun gash describes a wound or cut, so it makes sense that as a verb, gash describes the act of making that wound or cut, as in, people signing up for a knife skills class taught by a chef because they are tired of gashing their fingers when chopping vegetables.