Headset upgrade, problem finding headtube dimensions for my 2020 Giant ATX 3

You can measure this information yourself sooner than discover it here or elsewhere. When I've done the same thing on bikes I've noticed how little information manufacturers provide on things like this, I wish they gave more info.

You need to measure the fork steerer diameter at the top and at the bottom where the crown race seats, as well as the inner diameter of the headtube top and bottom. so four numbers in mm.

Then you need to know what kind of headset is required. It could be External Cup (EC), Zero stack (ZS) or Integrated (IS). Looking at photos it's not EC but you can detect what it actually is quite easily in person.

With this information you can specify the requirements, search for 'S.H.I.S code' for more detail, Park Tool's explainer is nice and clear.

Up to you if it's good value for money to replace such a new headset.