Hi Zack!
What is this 'GPSMAP76S'??? If I am understanding correctly, it plots grids on the GPS screen? If so, is this software or a separate GPS receiver? I have a Garmin Venture HC, would it be applicable to it? It would be a lot easier to look down and see what grid you are in on the display rather than calculate it using lat/lon.
The Garmin GPSMAP 76S is a discontinued GPS receiver:
https://buy.garmin.com/shop/shop.do?pID=183
I can make its main display show either my latitude/longitude or a grid locator. No need to convert latitude/longitude to grids or vice versa. Many Garmin GPS units, and some from other manufacturers, have this functionality.
In a quick scan of the PDF manual for your Venture HC GPS, the word "Maidenhead" - the fornal name of the grid-locator system - did not appear. It doesn't look like you can make your GPS display the grid locators directly. It does have a screen where it shows the latitude/longitude and accuracy figure, so it could be used if you took the pictures to document your location on a grid boundary or 4-grid intersection.
73!
Patrick WD9EWK/VA7EWK http://www.wd9ewk.net/