The main reason for that is because of the system the Leafs have always played for some reason. Pre-Babcock they were a run-and-gun team (stretch passes) and now with Babcock they're much better but still a stretch pass team.
When Gardiner has time, that means the other team has time. They're all set up to block all the stretch passes. What options do the D have (not just Gardiner)? That's where Gardiner's issue comes in. He tries to force a stretch pass even under those circumstances which is not ideal.
At the end of the day, if the stretch pass is there to make, go for it. If not, like you’re eluding to, don’t force it. It takes skill and timing between both the passer and receiver to execute it properly. I find many of the Leafs’ skilled forwards have the quick instinct to react, hockey sense to position themselves and foot speed to accomplish a stretch pass reception, if the opportunity calls for it. Like good wide receivers on a football team racing up the field giving their quarterback a place and time to make their throw. It has to be timed perfectly between both the QB and receiver. Otherwise it’s intercepted or incomplete.
Gardiner is like a QB who can work his way out of a pressure situation with a quick toss, run or handoff when under pressure, but when he has a bit more time to throw down field, he is a fraction of a second out of synch with his receivers, and the pass intercepted or iced. He also has no idea what play was called, and his teammates often have no idea what he’s doing. It’s a recipe for some random pretty plays every once in a while, but some really bad, costly plays as well.