Cannon items
The boots are very important as this is what makes you a literal rampaging monster in the battlefield. Since gloo's ult uses tremendous mana, demon shoes can help with that problem.
Sustain emblem
This set up is very helpful for gloo.
Battle Spell
This spell is useful for completing gloo's combos. As well as pushing enemies into your teammates to help you finish them up.
Dominant mid laner
Since gloo kinda flop in the exp lane, you can use him on the mid lane. This approach is very useful since gloo is a monster in the early game. If your jungler is an aggresive invader, you can help them steal the enemies buff for secured snowball. Also, you can help in literally every turtle, lord, and even team fights with this mid gloo strategy.
Best With Cyclops
Cyclops ult can lock down enemies in which gloo can utilize and lock down enemies for easier kills. Plus, cyclops insane damage and low cooldown synergize perfectly with gloo.
Watch Out For Faramis
Faramis second skill can deal insane amount of damage to gloo when in his split form. As much as possible when you're matched with him, avoid using your split form frequently. Only use it to lock down enemies and immediately use your ult to avoid faramis's second skill