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
t the top right corner of the
screen to send your Gmail message.
NOTE
You can change the message action after you've started the
message. Simply tap 
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.