i assume because true strike works with the other shots when you use flak cannon (need confirmation), but daed can't crit unless its the main shot. But thats just my speculation
This is correct, but you also have to look at another aspect of it: Mainly what damage items are available on the hero. When we think of "damage items", we think of stuff like Desolator and Deadalus. However on Gyro neither of these items are good, so what's next down the line?
Obviously Rapier exists, but we definitely don't want to get that. Monkey King Bar is so core on Gyro just because it's the highest damage item in the game that isn't reliant on being a Unique Attack Modifier, a crit, or anything of that nature. It gives +88 damage. Daedalus gives +81 damage, and then of course the crit obviously.
Gyro isn't particularly reliant on attack speed. Agility helps with his damage, but still his main source of attack damage is from his Flak Cannon, so you want items that work with that. MKB is basicallly the definitive item for that, even if there is no evasion on the other team. Gyro's damage output is the number of Flak Cannon charges, then goes down significantly. Firing these Flak Cannon shots faster rather than having them do more damage won't help you, since they are limited, so you want to maximize damage out of each one.