Yes this is the part you will require.

The majority of this is aluminium, with a steel bolt

Yes, this product would work with the GT CNC stem.

This stem adapter is for 22.2 or 25.4mm only. The 21.1mm version is an obsolete size now and not readily available.

This adapter works in a very similar way to a standard quill stem. However, instead of the handlebar clamp there is a blank shaft. This shaft is a 1.1/8th size and therefore allows a standard 1.1/8th stem to be attached. The adapter itself is tightened into the steerer tube via a 6mm Hex key in the top of the adapter. The same process as is used with a standard quill stem.

This stem adapter does not require any spacers as the Headset bearings are already preloaded through via the original threaded adjustment system of the headset. The bolts to clamp down the stem would be part of the stem and not this adapter assembly.

The adaptor will fit Compatible with 22.2mm and 25.4mm steerer tubes for 1" and 1/18th quill stems

yes this will work with the GT stem

No the quill mechanism will prevent you from running a brake cable through the steerer

it can not be used with the front break as the quill system will prevent the cable running through the stem and forks. It can be used with a rear brake system though.