yeah. and the point is that, that's the point. The guy I responded to essentially said, "dont be bad and get hit". But what kind of response is that? Half of the champion pool has no mobility skills to evade abilities like this. And while that isn't a problem due to their kit being geared toward that being an intended weakness, Gragas retains the ability to use flash to extend the range or even correct the skill being missed.
Which means standard predictions and manually running out of the way doesn't even apply to this particular ability because it can make a 90* turn in .1 seconds due to flash WHILE retaining the ridiculous hit box the OP demonstrated. Body Slam on gragas is a short range, linear skill shot ability that has a travel distance speed. Anyone that can get themselves out of range or around the dashed ability has utilized the abilities standard and intended counter play. Gragas overcomes this intended counter play by being able to freely use flash to change the angle, range, and speed of the skillshot at will. This may come at the cost of the summoner spell, but it removes the skills intended counter plays in return for often being able to net a kill. also, as i'll say again, bearing this hit box.
It's an intended flaw, being removed to exploit an enemies intended flaw, while being really generous about it. which makes it very overbearing.