It's not so much on the wrong but more on the
value for money.
GL vs full spec diff is RM15k, this is known fact. But what people don't realize what the RM15k gives you are:
1) BiXenon (not sure is HID or LED but white light) projector headlights vs GL halogen (yellow light) projector
2) More i-ActiveSense safety features and toys such as smart city braking, lane keep assist (some people need it while others hate this

), cruise control, rain sensing wipers, light sensing headlights, AFS (headlight turns with your steering)
3) bigger rims and better tyres (16" Toyo Nano Energy vs 18" Toyo T1S)
4) Halogen (GL) vs LED (full spec) tail lights
5) Paddle shift (GL don't have)
6) HUD
If you decide you don't need all the above, the GL is just fine.

By the way, not all CBU car build quality are good. I believe only CBU cars from certain countries (mainly Japan) are good. The previous Mazda 3 and current Mazda 2 are CBU Thailand which I feel is not very different from CKD.

also there is leather seat and electronic seat on hi-spec. air-cont panel also a bit premium.
mazda 2 also have leather seat and premium aircont panel, paddle shift and HUD.