This isn't specific to Google, but IP reputation was used heavily for IPv4... but there either isn't enough reputation for IPv6 or the systems haven't yet scaled to make sense of it (what works for the IPv4 address space doesn't necessarily work for the IPv6 address space, i.e. storing and looking up reputation and the slow changes in IPv4 ownership and the rapid burn-through of IPv6 ownership).

What people on IPv6 are getting is one of two things: 1) Harsher defaults or 2) No reputation system applied.

In the case of Google for email, it appears that it's harsher defaults.