The Excel macro in Cloud no longer has the "Show Grid" option and therefore it's not possible to show gridlines within the macro.
I've also attempted doing so using the markup available on the Server version of the documentation, but it's not possible.
In addition, the Excel macro is being rewritten, but on the new editor it still doesn't seem like there's a way to include gridlines.
Referring to this documentation and in tests on a known instance with the new editor.
Provide ability to show gridlines in an Excel document embedded on a Confluence page.