Ummmm, not quite.

As stated, the A15 head has smaller ports than the GX, but not all that much smaller & they are well matched to the engine for most street & a great many race applications.

The upside is that the A15 has larger valves & it usually comes with the rest of the A15 engine which saves a great deal of hunting around.

There are two A12 GX heads [actually, .... three]

The first is for the 1200 engine & it is drilled for the rocker oiling system that the 1200 engine used. The later engines fed rocker oil to the recess in the block that is also used for one of the center head bolts.
You can use the early head on the later block without any problems.

The second head was for the A12T [120Y GX] & it used the same casting as the 1200 but was not drilled for the earlier engines rocker oiling system.
This head should NOT be used on the earlier engine unless it is suitably modified to get oil to the rockers but will be fine on an A15, or any of the rear distributor engines. [1974 onward].

The third head was also for the A12T & is dimentionally interchangeable but has a slightly revised casting that removes a small boss on the distributor side that was previously used when drilling was done for rocker oiling in the 1200 head.
Naturally, this too will fit & work on the A15 engine but should not be used on the earlier engine without suitable modification.

To all practical intents & purposes, these last two are the same head.

Use the A15 head & you will not suffer any degradation in performance against the GX head & your wallet will be eternally gratefull