Open a gab post on your homepage timeline at the top.  Alternately,  by click/tapping on the pencil icon or using the comment option as a reply to another's post.

Good to know:
- The composer box has a limit of 3,000 characters per post.
- A character count indicator is available on each post composer box.  Located near the 'publish', the border will begin to fill in as you type to the maximum of 3,000 characters.
- Editing is an option found in the (ellipsis) 3 dots of each post box or just under a comment/reply.  Click/tap the 3 dots, select "edit" and make your edits.  Next click publish edit.   
- To open a conversation thread on an existing post, click the text, the time stamp or comment, when the a number is present.  The number indicates the number of comments in the thread.