When you respond to a Gmail message you can choose to type within the previous text. For example, if the previous text includes multiple questions, you can insert your responses immediately after each question instead of responding as a separate message.
Open a Gmail message and initiate your response action (e.g., Reply, Forward, etc.).
Tap the QUOTE TEXT checkbox below the Compose field to checkmark it, then tap RESPOND INLINE.
Tap where you want to insert your text, then enter your text as necessary.
Tap the Send icon in the upper right corner of the screen to send it.