Hi Travis - there has been some great advice here on how to get around different keys when sitting in with a band
It is a fantastic to have the opportunity to play and enjoy music with others. The best thing about playing with other musicians and with singers is that the sum output is greater than the constituent parts - in other words you can be a part of something that is creating music way beyond what you can produce on your own. So once you have the technicality of the key sorted the next question to ask yourself is 'What can I add that will enhance the mix of sounds in the moment". It is all too easy to sit in with a band and 'play for yourself' just as if you had the band on CD and you were sitting at home playing along. Like singing along to radio in the shower - you may be enjoying yourself but your not actually adding anything to the music that others are going to want to hear
So have a good think about the role your instrument is going to play for each tune and at every moment within that tune. Think about what you are going to add to the collective creative package. The golden rule is that if you cannot enhance the music at a particular point, or worse still if you will detract from the collective sound, then don't play.
The mountain dulcimer can do a number of different jobs within a band, rhythm, lead melody, counter melody, fills, starts and ends etc so have a good listen to each tune the band plays and decide how you can enhance each one.
Another issue for you may be how to work the instrument acoustically in a band situation. This can be a problem because the mountain dulcimer, played in contemporary chord melody style, carries far less acoustic volume than other instruments. I don't know how big the gospel band is that you are joining or what other instruments are part of the band but you may want to consider how you are going to sound balance your dulcimer in the mix. Personally, although you can work different keys from one instrument you may find it handy to have something like a McSpadden Ginger in your bag - a small bodied short scale dulcimer tuned up high to G. Sometimes a smaller dulcimer fitted with slightly heavier than standard strings wound up to a high tension will cut through a mix and also place you above the guitar and banjo in your own sonic space. You can then work the instrument more like a mandolin, providing choped rhythm and fills in the upper register. It is a very effective way to play in a band or session situation where a standard VSL dulcimer tuned to DAd would be drowned by the guitar (as they are both trying to fill the same sonic space - and he is bigger than you!!!)
Playing with a band can be a wonderful experience and can really quickly improve your own playing. With a little thought you can make the dulcimer an elemental part of the collective sound - and have a great time together.