To label an email, select a message then tap ‘Label as…” from the drop-down menu on the Floaty Bar. In the pop-up menu, select the label(s) you would like to use and tap ‘Apply’. Please note, you can add and remove existing labels to your emails in Gmail for mobile, but labels can only be created, renamed and deleted in the desktop version.
To label your emails on the go, point your mobile browser to gmail.com on your iPhone or Android-powered device. To make it easy to check your Gmail, try creating a home screen link. The new Gmail for mobile supports iPhone/iPod Touch OS 2.2.1 or above, as well as Android-powered devices, and is available for US English only.