The gauntlets I think you're referring to are the Forgemaster's Fingers. You're right that they convey a 12% bonus to smithing, however they are not truly related to Gloombound Mines. They are instead part of the radiant quest Find the Forgemaster's Fingers which can be started at any Orc Stronghold.
The reward for successfully completing this quest (and therefore returning the Gauntlets) is the privilege of being seen as Blood Kin by all Orcs in all Orc Strongholds. This gives the advantage of freely entering Orc Strongholds without encountering hostility and also to trade with and use their facilities.
However, killing the orcs and not returning the gauntlet will not prevent you from becoming blood kin in one of the other settlements, as there are several other ways to achieve the status:
- Complete the miscellaneous quest "Clear Kolskeggr Mine." After doing
so, report to Pavo Attius. Upon finishing the quest, Gat gro-Shargakh,
a fellow Orc miner, will send out a message to all of the Orc
encampments that the Dragonborn is now Blood-Kin, thus allowing them
to enter any strongholds.
- Bring a Daedra Heart to Moth gro-Bagol at
Understone Keep in Markarth, and he will send word to the strongholds
for the Dragonborn to be trusted. (Either changed by 1.9 or possible
- Speak to Ghorza gra-Bagol in Markarth. She will give the
Dragonborn a quest to find a skill book for smithing for her
apprentice. Once the book is found and given to her, she will then
send word to the orc strongholds that the Dragonborn can be trusted.
She will then also be possible to marry.
- Help Ainethach get rid of the
Silver-Blood mercenaries by persuading or bribing them to leave. DO
NOT get Ainethach to sell. Once they have left and Ainethach has
rewarded the Dragonborn, speak to Lash gra-Dushnikh and she will send
a message to the strongholds that the Dragonborn can be trusted.
the Dawnguard expansion, upon completion of A New Order, Durak will
mention sending a message to the Orc strongholds of the Dragonborn's
- Completing the first part of The Cursed Tribe allows
entrance to Largashbur, and fully completing the quest allows entrance
to all Orcish Strongholds, and the bestowing of the Blood Kin title.
- After collecting enough of books for Urag gro-Shub in the library of
The College of Winterhold, he sends word to Orc strongholds that the
Dragonborn should be allowed entry.
Having re-visited the Gloombound Mines to have a look round and see if I could work out a few more pros & cons for you, I'd overall say there's not much in the choice for Gloombound Mines specifically. You don't lose much for killing the residents of Narzulbur - Yatul and Urog are entirely unremarkable and Chief Maulakh only has the option for a lengthy brawl yielding only 100gp in profit. Bolar is a normal alchemy seller, so you would lose her services. Similarly killing Dushnamub would result in the loss of the blacksmith vendor and also would lose the opportunity to sell Ebony Ore at full price (insignificant if you're using the ebony for your own smithing).
There's therefore not many disadvantages to killing the Orcs, however there aren't many advantages either! The only one I can think of is that the Orcs in the mine will not move should you wish to use a vein. This can be resolved by waiting for an hour, however murdering everyone works too!