I've been plagued with cuts in tyres this year, and after a couple of gashes on brand new tyres after a few miles use, I decided enoughs enough and decided to mend my last gash. I just got an old tyre, cut off a piece that was far larger than the hole and trimmed all the corners off it to make it oval, applied rubber solution to both the tyre with the hole and the patch and allowed to dry then pressed them both together and they stuck solid. I then added some Ducktape over the top to help hold the patch in place and a bit of protection for the innertube rubbing on the patch. After about 3 months it is still holding.
But I would not do this if the gash was over maybe an eight of an inch and I would also ensure a repaired tyre is on the back.
These days now, I have decided to ride on cheapish tough tyres. ie marathons, Bontrager all Weather etc whilst solo and save the best tyres for fast club runs and events to try and keep costs down.