This happened to me, too. Apparently, the university that runs VPNGate has a lot of their own exit nodes now. If you look at the Operator's Name/message section, (either in the client or in the VPNGate website listing,) you'll notice the nodes are "for academic use only." It's possible Abema and the university cooperated to block those nodes, or something along those lines. If you're using the software client, you'll actually have to maximize the window where you select what VPN exit node you're going to use, I'd sort by country so all of the ones that exit in Japan are grouped together, then look for one that's NOT affiliated with the university. Since the university nodes, naturally, have the highest capacity, they're currently sorting to the top. I'm going to try and include a screenshot below, but since this is my first post, I apologize if it doesn't work.
I'm using a non-university node right now and Abema's working just fine :)