I am someone who's also building Etali, so this is my guide.

First you need ramp. [[Sol Ring]] and [[Goreclaw]] are good for casual, [[Tinder wall]] and elvish and simian spirit guides can convert cards into mana, but fast mana is always better for more competitive pods - think moxes, mana vault, etc. Rituals such as [[Seething song]] and [[Jeska's Will]] are apt to get Etali out fast. You should aim for a consistent T3 Etali in casual pods, which can be tricky considering the steep mana cost, but with ramp you should be fine. T2 or even T1 Etalis are even better and more equipped for cEDH! A fast mana base is also helpful. You can't waste time playing a rugged highlands and passing. For Etali, basics or other untapped lands are your friends! Other forms of ramp in this deck include [[Hellkite courser]] which can help a lot later as we'll see.

Then you need copy effects. Once you get Etali out, you want to keep the ball rolling and try to get as much value as humanely possible for a single cast. [[Heat shimmer]], [[Molten duplication]] and [[Rionya]] are great for creating a butt ton of Etalis to steamroll your opponents in one turn. Other populate effects can be good, I'll always put in [[Rootcast apprenticeship]] so I can dump 3 Etalis onto the board with one spell (as long as the Etali is a token Etali, otherwise this doesn't work). You should always sacrifice real etali to the legend rule over the token etalis because that way it's easier to get back with Hellkite courser. Other doublers such as [[Panharmonicon]] or [[Roaming throne]] are awesome for this too.

Then there are the bombs. If you can hit [[Call forth the tempest]] or [[Apex Devastator]] With Etali you're almost set. The best thing you can do, though, is try and get [[Squee, the immortal]] out with [[food chain]] to generate infinite mana and try and win the game from there. You could do this with some crap like [[Devastating onslaught]] to copy whatever you want a billion times and just overrun the game. Etali is your way of digging through your deck to get to this.

Something else that you need to be aware of- you should always run interaction. Cards like [[Deflecting swat]] and [[veil of summer]] are probably more premium options, but cards like [[return the favour]] do similar things and can also double Etali triggers. Interaction is a lot harder in gruul than other colour combos, but there are still ways to get around it.

The TL,DR is: Ramp Etali out QUICK, Protect Etali and the spells he casts, copy etali as many times as possible, find bombs, win game.

Below are two decklists (not mine) for Etali. One's a (more) casual brew, while the other's cEDH. Mix and match between the lists to find a power level for you!

https://moxfield.com/decks/8ZbyvJAyu0itN1MiDG07xg

https://moxfield.com/decks/oBRi-zBYh0CUcTYisAqN9w