Do you need to serve that over HTTPS?

EDIT: Alright, I figured it out, this page is out of date. Google services reply with this header which works:

alt-svc: hq=":443"; ma=2592000; quic=51303431; quic=51303339; quic=51303338; quic=51303337; quic=51303335,quic=":443"; ma=2592000; v="41,39,38,37,35"

No idea what these quic values are :) but here it is.

Those are probably supported version numbers.

yup, 4-octet version numbers for IETF QUIC (according to the draft)