Okay, so after some further investigation I think it's limited to FireFox developer edition
Seems as though the logs only appear in that browser, not in Chrome or Safari. The logs specifically come from The _ticker function in gsap-core.js and more specifically this line is where FireFox flags the log output but clearly it's not a console.log.
You can see from a simple codepen it outputs a lot of logs whenever GSAP does its thing in Firefox Developer - https://codepen.io/GreenSock/pen/mdPzJKp
manual || (_id = _req(_tick)); //make sure the request is made before we dispatch the "tick" event so that timing is maintained. Otherwise, if processing the "tick" requires a bunch of time (like 15ms) and we're using a setTimeout() that's based on 16.7ms, it'd technically take 31.7ms between frames otherwise.