First of all i like how you presented this idea. I feel like the roost part is really fitting for the champion, both for unique gameplay and lore parts.

Feels like powerfull anti-mage champion in top/mid and probably even support, certainly against morde adc or combined with kalista , thanks to the mr into utility scaling. Roost effect will need more balancing since it looks as an strong anti siege/dive ability (certainly with max 20s cd so you can use it every wave). Might be soft countered by zz'rot portal (which if it is would be great, zz'rot needs more love <3). I feel like the leaping time should be faster going into your righteous gust instead of a passive movement boost. I think ulting while roosting should be allowed since you can just leap off the tower and land right next to it (i assume this takes very little time). I'm not sure how i feel about bonus armor scaling of mr on bulwark might become too op against ap teams. I think the cc imunity on ultimate might be too strong late game. It cancels some hard counters galio's ult has at the moment though, like braum passive or xin q, so i can see why you add it. But i think in competitive lol having one good ult on the adc will result into a won team.

I think switching the roosting ability of w with runic armor of the passive would be better (you can use a seperate range/cd timer for balance). Might also be worth looking into making galio and the turret one entity during roost, to use bulwark on turret/yourself and make hitting a q more rewarding, need to look into who takes dmg then.

I feel that allowing to roost on enemy turrets might give him some more offensive possibilities, offcourse not from the 800 range but 100-200 range with some travel time, need to look into drawing turret agro and ifso then the bulwark + inhib turret effect, namely the free healing you might achieve or disable it on inhib turrets like azir. I think that it would be balanced since letting galio this close to your own turret is readable. Might be OP 1v1 though, a lot depends on his leaping speed.