I cannot even see our on-prem gateway in fabric.microsoft.com. There are no gateways or connections listed. If I use the old app.powerbi.com route, then the gateway and connections are all there and I can continue to manage them. In both cases, I navigate to the page from Settings > Manage connections and gateways. I can get round it for now, but it seems to be a bug.
We have seen on the internal platform that other engineers have reported this issue and submitted it. The engineers on the product team are now aware and will do their best to resolve the issue. I will update here if there is any progress, so please be patient.