Down the rabbit hole

Help if you measure the size of the group from the furtherest two holes up and down and same from side to side.
Try the holding your hand strait or on finger tips to get closer to point of balance. You can also try adding some weight to the back of rifle to make it more balanced.
Your junior are rifle probly does not have a much kick, that stops the gun moving as much. Target shooters will tune thier air rifle down in fps so the kick is less and more accurate.

Upload any pictures you like here. Maybe pratice and upload some target pictures too. We can tell alot from targets. If you upload a target and say every time I shoot it is on the target, but we look at the picture and the shots are all over the place and none are together we might go, that could be the pellet you use. (pellets make a big difference to how a gun shoots)

The 4 tins at bottom are the pellets my guns like. 2 in 177, 2 in 22. The top ones are pellets that I've tried that don't shoot as good in my guns, and there's still about 5 other brands on top of that I've tried that weren't as good. None of them are wad cutters ( flat nose pellets that target shooters use)

You can try putting the rifle on finger tips at the front. It means your hand is straiter and that means your fingers are supporting the rifle further forward. It is how I use to target shoot, I don't like it for hunting, and don't use it much anymore because I mostly hunt so pratice how I hold it hunting.

Your arm is probly not use to holding a rifle up. You will have to just keep using it and it will get stronger.
Some rifles are heavy. I've swapped a very good rifle for another lighter very good rifle, because it was to heavy to carry hunting all day for me.
If your arm gets tired have a rest.
GAMO are light rifles. Some people love them some people don't.
They are not the best rifle for target shooting,in my opinion .
You may have to go through a few rifles to find one you like. Don't give up on using the artillery hold as it very difficult and it will take a while to get use to it. In the beginning it will not be very accurate because you haven't practiced it.
If your arm gets tired have a break.
When I did target shooting more years ago than you probly are old, one of my shots took 21 minutes. I stopped and rested, I had to stop and breath because I had held my breath for to long more than once. But it worked. I put 5 shots into a group that was barely bigger than a pellet hole at 10m. ( they did ask me to shoot a bit faster after that shot)
Target shooting is about technique and presison. It is also a about the gun if you want to compete. You will need a good gun or make your gun a good gun by changing things to work better.
At the moment you can pratice your shooting technique with your guns and get that right, but probly will have to get better guns to compete in target shooting. For fun target shooting they are ok.
It takes a lot of time and a lot of learning to shoot springer air rifles.
It takes a lot of pellets.
You are right to ask stuff, you need to, to learn.
Its just the beginning, don't rush, don't worry if things arnt happening fast it takes time.