GAB is a viable, free speech oriented, Twitter replacement. It has clone functionality for Twitter (max 300 char posts, #hashtag oriented), with some of Facebook's functionality (GIFs and images). Looks better than Twitter. Functionality still under development.
*Users: 215,000
*1 year old
MINDS on the other hand seeks to do the same thing regarding Facebook in particular, offering the same functionality. I see Minds and Gab as not mutually exclusive.
*Users 1 million +
*2 years old
How they both compare to Steemit is now relatively easy to see, Steemit is unique and different from them in just the same ways it is different from Facebook and Twitter, in terms of functionality, but more or less the same as them in terms of free speech. However, Steemit is blockchain based, hence inside corruption may not arise in the future in the same way either of them might allow at least in theory (though there is no saying this will happen).
Hope this helps.
RE: What's the difference between Steemit, Minds, and Gab?