As long as you have space under 5m for Gunnery V, then of course, you can pick that or anything else from the 20m SP list.
It seems the OP is at 5m however and expects to be able to train ‘old’ Alpha skills just because he used to be able to.
They become the only way to train more skills, be they ‘new’ Alpha skills or ‘old’ Alpha skills. Essentially new and old is irrelevant and a thing of the past. You can have 5m out of the 20m for free, and 15m more via Injectors.
I’m assuming in the case of the OP, he was once an Omega, and trained some (at the time) Omega only skills that are now Alpha and count towards his 5m allowance. This is how you could end up an Alpha today with 5m in skills but not all of the old Alpha skills.
Or maybe he already had time today to Inject some ‘new’ Alpha skills and hit 5m and then tried to train Gunnery V believing he had some right to do so because ‘old’ Alpha, which makes no difference now.
…which does indeed support one of the above scenarios, that he’s at 5m and trying to go back and train ‘old’ Alpha skills, when actually there is no such thing anymore and ‘old’ or not is irrelevant if you are at your ‘free’ cap.