First, get the specifications for the malt you want to add. You can find a fact sheet on the MaltGems product you cited here: https://maltgems.brewingwithbriess.com/documents/2020_SynergySelectMaltGems_SS_5-21-20.pdf
Now in BeerSmith, open up your fermentables ingredients listings. Click on 'Add new...' on the tool bar to open up a blank fermentables listing. Fill out the information you can from the sheet. Don't sweat the small details as the only real information you will need is potential or yield, % moisture, name and (if you plan on using it with a lot of adjuncts) diastatic power. If a malt does not list a diastatic power, it is usually because it no longer contains active enzymes.
Close it and add it to your recipe.