Hi John. Using the Colors or GP Premium Add-On you would go to Appearance > Customize > Colors > Header Colors > Background > Select Color and remove the hex value.
If you want to use CSS this should do it:
.site-header {
background: transparent;
}
Colors Add-On: https://generatepress.com/downloads/generate-colors/
GP Premium Add-On: https://generatepress.com/gp-premium/
Adding CSS: https://generatepress.com/knowledgebase/adding-css/