To reply to or forward a message
Open the message you want to reply to or forward.
On the message header, tap the Reply icon
or
tap the Menu icon 
, then tap Reply all or Forward.Add others to an ongoing conversation by replying to all and then adding the new person’s address(es) to the reply.
Tap the Send icon
(at the top of
the screen) to send your Gmail message.
NOTE
You can change the message action after you've started the
message. Simply tap the action icon 
in the message header next to the current message
action and tap the one you want. For example, if you chose Reply but
decide you want to Reply all instead.