Depends what kind of attack they use. A few years back, reflection attacks were all the rage, and it was cheap as fuck. But I can't say how many Gbps they delivered.

They'd usually market this as "stresstest" service, and you could pay by Paypal etc. Curiously they were all behind a Cloudflare-anti-DDoS wall. I still wonder if Cloudflare knew/knows about them, probably good for their business.