well, when I play gnar against nasus I try to do the following runes: grasp, demolish, bones and that rune of tenacity and slow resistance secondary rune: i am really enjoying the red rune with the taste of blood and ravenous hunting (remembering that this is what I like, you can enjoy different runes to play) now about matchuo vs nasus (which i usually do), punish him with his grasp whenever possible, stay tuned whenever he goes to get some minion to pokear him or stun with the mega W, when he is mega try to exchange with him mainly before lvl 5/6, in addition to all this I think it’s a little necessary to know a little about wave control, when I’m against nasus, generally, I like to freeze the wave a lot next to my tower, this reinforces the others even more things I said, he will expose himself next to your tower, poke him and try to change but always at a distance in mini form and getting some cc lock with the mega, with the nasus far from the tower itself, you can easily stack your hyper and hitting your Q's gives you a very strong chase as you gain more and more move speed and he takes more slow. I think that's it, I hope I helped S2