I have reached KOG 5 months in a row with F2P deck. I have never spend money on this game before but it will take you time and practice to make you reach KOG. Anyway, I am happy to share my secrets of how to reach KOG without any money. Let me know if you guys are interested.
There's no secret, anyone can start the game right now and spent his gems on the last box and build an Alien deck and reach KoG,and there are other decks that you can build without using many gems and reach KoG.
Sure, first you need to know yourself if you are a aggresive or defensive player.
For those who are really F2P, i would recomend to buy staple cards. Cards that can guaranteed you to reach Legend. Because these cards are good for both offensive and defensive.
Cards I recommended to invest are: sergent electro, WOD, Xing Zhan Hu, sphere kuriboh, spell&traps removal