As a mastery lvl 5 and who knows how many games I have on Gnar, I run 21/9/0 masteries on Gnar, with standard ad marks and quints, armor seals and mr glyphs.
For Builds, it really depends on the matchup for me. I tend to rush  , but sometimes I go for  if my lane opponent is an ap bully like or  . After I finish Phage, I get  or  depending on the enemy team comp. After that, I rush out , followed by.
If I am very ahead and my team needs more damage, I will go  next. If im not super ahead or my team needs tank. I will go .
After that if I got , I will go . Some other optional items I tend to choose from, depending on matchups and how the game is going, after this are:, , , , .
I also don't agree with the  build, I don't like that the mana is going to waste. I definitely prefer the  to it. Sometimes , , are viable as well
Like I said, it all depends on the matchup. Don't get stuck to one build every time. My build is different every time, except for the few core items I get.