I'm having sync issues over WiFi since the update to Marshmallow. This is similar to what others and I experienced with Lollipop, described here LG G3 lollipop update - no app or email notifications while on WiFi.
When on WiFi, I'm not getting Gmail push notifications and new emails are not shown in the Gmail app until I manually refresh it. Google Calendar and Contacts aren't syncing either. Everything works correctly on 4G.
Is anyone else experiencing this? I haven't had a chance to try some of the workarounds from the Lollipop issue, I've just stayed off WiFi this morning.