Set the textwidth option first, e.g. :set textwidth=77

Then gq will reformat the lines you want.

For example,

• select a block of text -> gq
• you’re in a paragraph and want to reformat the whole paragraph -> gpip
• reformat the next 10 lines -> gq10j

Here’s how I configure this in my .editorconfig to set a textwidth in markdown files:

[*.md]
max_line_length = 77