When I went off the trail on my last hike, I noticed that grass got into my boots very easily because the toe is not gussetted like in other boots. And gaiters don't cover low enough on the boot to keep debris out. The boots are really comfortable and supportive and I don't want to get new ones. Anyone have any tips to add a gusset?
Welcome to the site.
Without knowing which sort of boots you have it is a little hard to provide specific advice. Hiking boots in general are made with materials that are designed to be impermeable to debris. Regular "sports" boots like basketball boots are designed more to be light and breathable (to prevent sweat build-up, modern hiking boots do this through breathable layers like goretex), and are not generally suitable for hiking - for one thing they will break down much sooner than a pair of hiking boots due to rough terrain.
There are some design features that you should look for in your boots that might help:
Unless the tongue of your boots is gussetted to the upper, you won't be able to prevent grass and debris from entering your boots. If yours don't have a gussett, take them to a reputable shoe repair shop and see if it is something that can be added.
Look at your lacing pattern, make sure that it closes firmly over the front/top of your foot, so that there is a smaller gap for debris to enter.
Look for gaiters that hook onto the lowest set of laces. These aren't just for protection of socks, but will cover the boot leaving just the toe and a small proportion of the side exposed. Generally this is a feature you find on knee-length gaiters, but not shorter ones.