>>> I've just run a test in my group by replying to a post via Gmail and found out that the Like is recorded only if the reply doesn't quote the original post (i.e. you trim the post "tail")
I also tested this a couple of days ago in a beta/docs topic, albeit not from Gmail but from my local Thunderbird app. One can either reply to a message with +1 (or Like) as the only message body text, or reply but leave the quoted text in the message below, and add +1 as the first line. In both cases the like was recorded, I guess with the non-moderated stipulation J mentioned (which I didn't know but it's good to know)
>>> If you reply with +1 as message text and leave the original post below, your reply apparently vanishes in thin air and does not show in the group.
This is by design, using email, you can either like a post or
reply to a post, but you can't do both in the same reply.
Correctly-formatted email Like "replies" get recorded as Likes to
the replied-to message/post but the message itself is not entered
in the archive. (again I guess with the non-moderated stipulation)