In your particular business--an art gallery--there is some considerable risk. For example, contract disputes with artists/buyers, liability for damaged works, a rogue business partner who makes unrealistic promises to buyers/artists, and so on.
All these things can--if you are unincorporated or a general partnership--expose your personal assets to loss.
If you are starting this gallery with a partner, I strongly suggest that you form an LLC, and NOT a general partnership. Otherwise, if the relationship sours, you can be held personally liable for actions of your ex-partner made on behalf of the partnership.
Furthermore, if you are going into business with a partner, you need an Operating Agreement.
Don't skimp on this!
You can save a couple bucks filing your Articles of Organization yourself. However, you cannot draft your own Operating Agreement (unless you happen to be an experienced business lawyer in addition to your other trade).
Either find yourself a trusted attorney, with experience in LLC Operating Agreements, to draft one for you.
Or, if you form your LLC through Legalzoom, they include a customized Operating Agreement with their formation package.
It's a very good deal. If you require an attorney's customization beyond Legalzoom's Operating Agreement, rest assured that by having a document to start with, you will save a significant amount on the attorney's revisions.