There is a way, kinda… I am assuming you want to go from this:

to something like this:

Maybe someone has an easier way, but if you don’t mind doing this by editing the theme, you can add a transparent border to the bottom of the budgie panel.
border-bottom: 3px solid rgba(0, 0, 0, 0);
Disclaimer: I’ve seen that you make themes and have some on gnome-look, so I realize the rest of this is not necessary, but I figure I would throw in the details in case anyone else is interested.
If you are using Pocillo for example, you can edit the file:
/usr/share/themes/Pocillo/gtk-3.22/gtk.css
Alternatively, you can copy the Pocillo theme to ~/.themes/ and edit it there instead.
Down around line 4796, you can change this:
.budgie-panel {
background-color: #383c4a;
color: #FFFFFF;
font-weight: 500;
}
to this:
.budgie-panel {
background-color: #383c4a;
color: #FFFFFF;
font-weight: 500;
border-bottom: 3px solid rgba(0, 0, 0, 0);
}
Change the theme, then change it back and it should have the gap now.
The big problem with this way is you have to do it with every theme you want to use (there is 6 variants of Pocillo alone) and you have to modify the gtk-dark.css file also if you want to turn on the “dark theme” switch in Budgie settings. Also, some themes are compiled, so they can’t be changed this easily.