Hi Simon,
It’s a default spacing setting in GeneratePress. Most widgets in the Footer and sidebar are stacked vertically. There’s code to add spacing between them.
Since the Social Icon widget is stacked horizontally, the issue occurs.
We highly advise using GenerateBlocks for creating Social Icons in GP though.
However, if you don’t want another plugin installed, Leo’s code should be the best approach as a fix. You can simply add it through Appearance > Customize > Additional CSS in your admin dashboard.