As a "Berniecrat," I couldn't tell you. I notice the seemingly wide support for her as well, particularly among YouTube commenters and do not fully understand. It makes even less sense considering that Gabbard, thus far, has not given any indication that she is even interested in running.
She is popular because she does and says things that people like. People don't like PACs, so she says she won't take PAC money. People don't like NSA survelliance, so she introduces a proposal limiting it. What is so hard to understand? This should be 1+1=2, yet we have 100 Senators and 434 members of the House who don't.