Hi Jusung,
It isn’t that there is spacing.
It’s more because the Container isn’t taking full width. What Container width do you have set for the Container Block? Reference: https://docs.generateblocks.com/article/container-overview/#layout-%E2%80%93-standalone-block
If it’s not full-width, can you try setting it to full?
If that doesn’t work, try adding this through Appearance > Customize > Additional CSS:
nav#generate-slideout-menu > .inside-navigation > .gb-container {
width: 100%;
}