Is there an extension/plug-in for extended grid options?

[Blighty]

(04-13-2019, 12:34 PM)Marscaleb Wrote: When I'm using the grid, it will only display the same gridlines at its designated spacing without any indication of "major gridlines." 

None that I know of.

But there is an easy workaround.
Create a transparent layer as the top layer of your layer stack.
Then Filters > Render > Pattern > Grid
For example, if your gridlines are set a 100px intervals, create this second grid at 500px intervals and set the colour to red.
Remember to switch off visibility before exporting, etc.