Focus Tank!

But what if the enemies' adc is afk and they only have tanks? kappa.

Multiple issues with g2. For one, they relied almost exclusively on their adc for engaging fights when ashe arrow is unreliable. Instead flank with lee and force enemy to run towards one side of lane/river so they have to choose between getting hit by lee sin flank engage or by the ashe arrow. Alternately, sacrifice your zyra to flash forwards, root, then ult. Either you kill one person, or you get a teamfight. Don't pick renekton unless you are planning to repeat gank and snowball him. If you do pick renekton - build instead of second. Sure you miss out on 450hp and damage is comparable if you ignore the %hp damge on bork. If you factor that in, it is much better than titanic for damage AND you get lifesteal which mean's galio's base damage chunking some of your hp out can just be ignored and healed up via lifesteal. You have more kill potential 1v1. And with empowered w in early teamfights, you basically flash w combo and delete any enemy carry. If they blow all their cc and exhaust to stop you, you still do a ton of damage and open that carry up to being deleted by shockwave. Plus, you are probably guaranteeing a 2+ man shockwave since enemy team clustered around you to peel/cc you. At worst, you trade 1 for 1 toplaner for enemy carry. At best, you live despite enemy cc and exhaust and get off some respectable damage to other enemies after killing the adc or mid - allowing your teammates to clean up especially since galio is probably peeling you this whole time instead of getting in your teammates' faces. g2 just settled for too small of gains/trades instead of recognizing their best opportunities and forcing for more. You can't be afraid to trade one teammate to guarantee either a trade for an enemy carry if not a won teamfight.