We are phasing out Zendesk. Please see Discourse instead: https://community.getmailspring.com/t/adding-removing-gmail-labels/250
To add Gmail labels to a message, click the label dropdown while viewing a Gmail conversation. Type a new label name to create a new label, or select an existing one from the list:
You can also remove labels from a conversation by hovering over them in the message conversation view and clicking the small "X" that appears:
Note that the label picker only appears for Gmail accounts - if you're viewing your unified inbox, you may have Gmail conversations side-by-side conversations from other accounts that do not support labels and the label picker may only appear part of the time.