Hi Gabriel,
I checked the front end of your website and I found out that right before and after the start of the main content there is an empty P tag in the HTML code:

That is not part of our theme and it seems to be a problem caused by customization. And as the P tag has a default margin you see the gap in question.
The empty P tag is a side effect of having block-level elements such as DIV tag inside the P tag. Whatever you put inside a P tag should be inline-level such as SPAN.
I removed the P tag from my browser temporarily and saw that it fixed the issue:
That is why you need to check your customization and find a wrapping P tag and change it with DIV.
I see that you use a Child Theme and you might have the customization there. You can test by changing the theme from Child Theme to X and see the result.
Thank you.