You can customize the vertical spacing between paragraphs in your Figma text layers by opening the Figma text properties panel, then adjusting the Paragraph spacing value under the Basics tab. This will determine how much margin-bottom to add to the <p> tags inside that text layer's content when your HTML email is generated.

Please note that if you've added any extra paragraph lines (eg. pressing Enter or Return twice to create a double space), the empty extra paragraph will still be included, and any custom paragraph spacing will also be added as margin-bottom underneath that, too. You can optionally remove these extra/blank paragraphs (as per the video above) to have finer control over the paragraph spacing in your text instead.