If it were me I would retire the tire. Booting is a good short-term fix to get yourself home. With that being said though, I imagine if you made a good patch out of an old tire, and glued it well over the gash, you could get some more miles out of the tire. But, I wouldn't do it. The risk of a blowout and subsequent accident just aren't worth it to me. You might want to invest in tires that are more slash proof next time. I have had great luck with schwalbe stelvios for being light, fast, and puncture resistant. Other folks swear by continental gatorskins.