Unfortunately, it is not easy to get an expungement in Indiana. Indiana is willing to expunge criminal convictions but only when they have first been pardoned by the governor.
Pardons are free to apply for and you can do it yourself without a lawyer. In order to get the pardon, you would have to apply to the Indiana Parole Board which does the screening for the governor. Their form is downloadable on line. You can obtain it at the bottom of the page I'm linking you to.
The Parole Board's form makes it clear that this matter could take a year for processing and that pardons are not granted all that frequently. Still, it's something that you can do yourself, and there's no reason not to give it a try. If you can get the pardon you should then have no problem getting the expungement.
There is one other way to get the misdemeanor off of your record and that is to retain a lawyer and have him petition to reopen your case and dismiss it in the interest of justice. A judge always has power under principles of equity to grant a petition like that even when the law doesn't provide for a dismissal. He or she will grant such a petition if it is necessary to prevent an injustice from occurring. So with a particularly compelling argument, if the judge is willing to hear the petition in the first place, it could happen for you.
Courts are sparing in their use of the equity power and many judges will not touch this at all, but it is something he could explore with a local criminal lawyer who would be able to tell him how viable this would be and whether your prospect of success would be worth the money you'd have to lay out to get it done. This woudn't remove the case from your record like an expungement would but it would dismiss the conviction, which would still be an improvement.
Apart from that, unless your case was a misdemeanor conviction for domestic violence, even with this old conviction on your record, it should not get in the way of renewing your gun permit.