How effective is baking soda against gingivitus?

Baking soda. Baking soda is somewhat abrasive and as such should not be something i would recommend brushing with every day, however it does have an acid neutralizing affect which could help prevent cavities, yet a good Fluoride toothpaste is likely more effective and less abrasive to your enamel. There are many antibacterial mouth rinses available that are effective for gingivitis, but no sub for effective hc.
What is important. Proper brushing and flossing after at least all meals is the single best way to prevent and treat gingivitis. Routine dental professional cleanings are necessary to remove any hard deposits of calculus that form on teeth as none of us are perfect and can always do a 100% great job on a daily basis. Everything else is secondary and while it may help is unnecessary.
It's not. While baking soda may have very slight ant-cavity properties, it doesn't cure gum disease and is more abrasive than toothpaste. Start with a professional cleaning to remove hared accretions and clean areas you're not routinely reaching. Have Dentist/Hygienist craft a personalized oral hygiene program for you. Research shows baking soda ineffective.
Not very. Years ago it was thought that baking soda and salt was a cure, and it was found to not be. And you're not getting any Fluoride protection from the soda either. Use a toothpaste that has baking soda in it if you would like.
Mild abrasive. Baking soda is sodium bicarbonate. It is a mild abrasive that may help to remove plaque. Some think that the salt, the sodium may help to reduce inflammation. Other say that the abrasiveness helps remove plaque and the helps more than the salt. Either way, if it helps to to clean your teeth, carry on.
Gingivitis. There are some baking soda toothpastes. Gingivitis is inflamation of the gingiva. If you look at the edge to the gingiva line with tooth you may see a tiny red line (that's gingivitis), if it bleeds is more advance, and sometime it get sinfected. Good oral hygiene (brushing, floosing, eating healthy). See dentist for oral exam and education.