I'm not going to be a guru in this arena, so I can't throw out hard number on what CPU and GPU speeds to hit. I can through out a smattering on my thoughts from what experiance I do have.
What I do want to ask is do you plan to dock to a screen for gaming? You can probably save good money by keeping with those 15" laptops if you are only going to plug in to a monitor, but I personally would get a 17" if you are going to be gaming straight off the laptop. I saw you were thinking of doing that with light gun games, but is that going to be the normal setup?
GPU is going to be much more important than CPU, but from what I am hearing, those Ryzen processors are soon going to be replaced with a much faster CPU that won't cost any more. That might also work in your favor, as I am hearing there are some great Ryzen discounts going on to clear out inventory. You might also want to wait until the new processors come out. For reference, my CPU on my gaming tower is an AMD from 2012, but I have a newish 1080ti graphics card. There is no game I can't play at high settings, but my FPS is generally limited to 30-60 FPS depending on the game due to CPU bottlenecking. Doesn't boterh me yet, though. If you are going casual, I would focus on the GPU. As long as your CPU is decent, you should be alright. Just think about future proofing so you get a number of years from your rig.
For reference, I don't have a gaming laptop, but I do 4k video editing on my machine for work so I am sure it would be an absolute killer gaming rig. It is a Lenovo P72.
https://www.cdw.com/product/lvo-p72-i7-8850h-512-16-w10p/5272472 It burns! They do cost a massive premium for their quadro GPUS, which are designed to not overheat under for long-term high graphics workloads, which is the situation of what happens when encoding video for hours on end.
I also recommend getting a primary SSD and a secondary HDD so you can benefit from quick boot speeds and still have large mass storage for games, and maybe house music and movies. Pay attention to internal drive bays as well as ram slots. I got a couple M2 slots on my and a 2.5 bay, as well as 4 ram slots, and have been slowly expanding as needed. I'd say as long as you have both an M2 and a drive bay, you should be set. Laptop manufacturers love to pre-fill all ram slots with small size sticks of ram, which forces you to remove a stick to upgrade. Stuff to pay attention to!
Good luck!