I see the focus that this deck is taking and i like it. My only thing is that you should probably add some basic mountains, at least 5 and some
evolving wilds. Last thing you want is to draw into Omnath and have to wait like twelve turns before you get any red mana floating. Also try playing
retreat to kazandu so you can keep putting +1/+1 counters on your
undergrowth champion.
Earthen arms and
dragonscale boon would also be good additions for this deck. If you add some
akoum stonewaker,
valakut predator,
reclaiming vines, and
slab hammer then you can keep repeating the landfall triggers, keep pumping out elemental tokens , and take a more aggressive approach to the enemy. Personally I'd add some black and make this deck Jund but if you wanna keep it Gruul then make sure you have some creature recursion cards like
restore.