What to do with excess Great Generals in G+K?

You could use an unprotected general to lure out an enemy unit. On destroying that enemy unit, it could make the difference in a successful offence or defence campaign.

Delete them, as they cost you money.
Proof
Two generals and state of finance.

After Deleting two generals. Saving 4G @ 2G per general.

I honestly find that, prior to the Modern Era and the advent of air bombardment, Great Generals in excess of the number you need to cover your entire invasion force in the 15% aura are best served to plop down Citadels outside of cities you are sieging that are particularly tough to crack, assuming you have nearby territory. Not only does this give a +100% Defensive Strength boon to whoever stands in it, it gives you an area of your territory that you can have units in, which if you have units with the March/Repair promotions, means that you're getting +20 HP per turn versus +10. This can help keep your siege weapons and/or archers far healthier in long wars.

If you're playing Sweden, you can always just pop them into City-States for the +90 Influence bonus.