At Internet.nl we're seeing IPv6 connection issues on TCP port 25 (SMTP) to mx[1-4].smtp.goog. Either 100% DROP (so no TCP connection) or ⅔ failure to setup connection. At least from AS20857, AS20860 and AS41887.

Can somebody ping Google/Gmail, it seems to be on their side.

Verify it yourself with: $ nc 2001:4860:4802:32::97 25

Replace 32 with 34, 36 or 38 for MX 2, 3 or 4, optionally use telnet / netcat / socat instead of nc. Please note in all cases ICMP (ping6 / traceroute6) does work.