When you say "guitar tab" do you mean six-string note-by-note tablature? Or are you looking at the leadsheets and chords-with-lyrics sheets that people on the internet call "tab"? The chordsheets are no problem. As Ken said, a G chord is a G chord. We can explain more if that's what you're looking for.
Actual tablature is tricky. It's not just that a guitar has 6 strings tuned different from a dulcimer. It's the chromatic frets. Unless your dulcimer is chromatic, you could figure out how to map every note on the tab to the corresponding note on dulcimer and still not be able to play it because you don't have frets in the right place.
Look into tab software like TablEdit. I think there's a way to enter guitar tab and convert it to standard notation. Maybe there's a way to convert guitar tab to dulcimer tab. Hopefully a TablEdit guru will check in here.
You might get somewhere with ukulele tab (4 strings) if you set your dulcimer up with 4 equi-distant strings and tune them DGBE.
Another option, and perhaps the most fruitful one, is to buy all the dulcimer tab books you can find that have contemporary music (there are some). After you've played enough tab, you'll get a better idea of how this works and you'll have more success at figuring songs out by ear.