I can not speak for other galleries, however, we do not accept unsolicited submissions. Most of the artist portfolios that we end up seriously reviewing, come after a few years of networking, following and getting to know the artist and the purpose and intent of their work. If we have become intrigued enough, we then may request a review. My advice to emerging, aspiring artists is to get out, network, attend openings, make yourself seen. Apply for competitions, grants and artist residencies. Network, network, network. Launch a website that has images of your work, bio, statement.. etc.. Use Instagram and Facebook, "Profession Pages" to post information about your work and images, not your social and political views or rants. Eventually, the gallery owner will become intrigued and want to know who you are and look up your work. The old school way of sending unsolicited submittals if frowned upon my many advanced tiered galleries. They will assume based on the large number of bad art that is sent to them, that you are simply one more. Know the gallery! So many times we get solicitations that do not even come close to the program we are showing. Step outside yourself and really analyze the programing of the gallery and whether or not your work is compatible. However, to answer the question that you originally presented.. should you chose to approach a gallery: A cover letter of introduction and a link to your well designed website will at least give most galleries a taste of what you are doing and the type of work that you do.