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 5,000 characters / post.  Pro subscribers -  1,500/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 5,000 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 counter (number) is present.  The number indicates the number of comments in the thread.

Posting options:
Post to timeline - If you've joined a group, when you click the 'Post to timeline' option box, you'll see a list of the groups you've joined.  Select a group to post to that group.
Add context -  When creating a post, click 'Add context' to add a bit of fun to your post.

updated 04/04/2024: character limits, context options.