It was the council of Nicea in 325 Anno Domini that much of the 'other gospels' were lost and/or discredited. That is when the politics of the the Roman emperor Constantine forced the different groups that called themselves ‘Christian’ to consolidate.
I fear much was lost at this time and modern religious scholars become archaeologists and must pick through the trash heaps of the holy lands to try to glean anything.
New Age nonsense. Nicaea didn't say anything about any "other gospels". Some of the spurious writings you're describing hadn't even been written yet.
Other than historically, what of value was lost?
Theologically, nothing. nOT OF VALUE.
Jesus and the early Christians used the Old Testament.
Or, when the Holy Spirit guided mankind through a minefield of errors.
A significant myth. Christendom was fighting these false Christians and their fake Gospels from day one, and yet affirming our own work-- if not explicitly giving a list (though Papias lists 4 Gospels), then at least by quoting the vast majority of the New Testament even as soon as before the 1st century had even concluded.
As for looking into "dust bins." If you want to know what you're missing, these false Gospels are indeed available. You can go read the Gospel of Thomas, for example, and discover that it was likely written by a retard without a Jewish background, if you compare what it says and its qualities with the very Jewish New Testament.