you are right.
I should make them of my own and i am trying it.
It's called smspva.com, I used it 3 months back to get a couple of Gmails verified but the downside is that the gmails get locked after some time and you maybe required you to re-verify them in order to reactivate them.
You can verify Gmails using Russian Numbers which are worth $0.1 per sms verification per email(You need a good private VPN or else you will have trouble getting the sms verification codes), your money will get you roughly 35 Gmails if you are successful.
You can research about smspva.com before using it. I am not affiliated to them.
Even if you are using an VPN, you will run out of IP's quick as Google's anti spam bot kicks in and detects that you are abusing the system. So you will need a bunch of VPN's. I've tested some VPN's in the past and they don't come with 100 IP's at all which is what you would realistically need to do this right.
You not only risk that Google locks the account but permanently deletes it which would be a disaster since you couldn't access it ever again.
Honestly don't do this for anything of value. Can you say why you are trying to create 100 accounts? And why would you need GMX specifically? It's pretty trashy. They delete inactive accounts so quickly. Satoshi got hacked that way.