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.
Compose your Gmail message as necessary.
Tap the Send icon
to send your Gmail message.
NOTE
You can change the message action after you've started the message. Simply tap the current action (at the top left of the message header), then tap the one you want. For example, if you chose Reply but decide you want to Reply all instead.