Siakam wasn't close to where GG is at the same age. It sounds like you're trying to compare the skillsets of an 18 year-old to someone who blossomed when he was 25. GG was widely considered the #1 prospect in his HS class before he jumped a year.
In terms of physical ability, even now GG has better balance, better body control, a better handle, better bounce and is shiftier than Siakam. It enables GG to score in more situations, including mid-range and off movement. Siakam's defense is aided in large part due to his length. He as a 7'3" wingspan, which GG won't ever have. GG is going to come into the league needing to learn how to play defense pretty much from scratch, but that's the case with a lot of one-and-done.
In any case, I've seen your takes on GG all year, and I don't agree them. Quite frankly, I'm trying to remember the last time I was impressed with one of your takes in this thread and nothing is coming to mind.