Bottom line, I think it would be a mistake to rely on ICMP echo requests/response for proof that a gateway is there or not. Our local repeater coordinating body requires coordination holders to update their record once every two years (or maybe it's three years). It's as simple as logging in and clicking a button indicating that their information is current. The database sends automated reminder emails when the update is due. It has worked well for many years and is not onerous for the administrative/system staff or the repeater owners. Something similar seems like the way to go.
This is indeed how the portal currently works.
Chris