No real tricks or secrets in grouting that I am aware of. Grouting of "tumbled" travertine isn't as difficult as grouting "chiseled-edge" travertine.
The use of a bag would probably be the way to go but have never used a gun. Go slow and don't get too far ahead of your ability to "dress and clean" the joints as it relates to the curing of the thinset.
More often than not, people that buy unfilled travertine are in fact buying the holes that occur naturally, and of course want to maintain the voids as a finished asset of the tile. My school of thought on that is that (with floor tile) these "holes" will in time collect undesirable alien matter. If moisture routinely gets into the holes it could be a source and breeding ground for mold and bacteria. I personally like to see the holes also grouted. The contrast of the grout is in my opinion enough to still "show off" the holes and they remain an asset of the appearance.