If the name of this learning resource made you think of Gengar the Pokémon, you're not alone. I did too! But as far as I can tell, this webapp has nothing to do with Pokémon. Gengar Podcasts is a website that provides transcriptions of podcast audio, so you can read along as you listen!
At the time we checked this resource, there were eight Japanese podcasts/radio shows available on Gengar Podcasts. It looks like a ninth is being processed: Let's Learn Japanese From Small Talk!. It looks like you can submit podcasts to be processed by Gengar Podcasts, so if you have a favorite Japanese podcast, you can try to get it transcribed using this service!
The transcriptions themselves are not perfect, but it's still nice to be able to read along as you listen, even if a word is missing here and there. Japanese transcriptions lack any punctuation though, so it is very difficult to tell where one sentence ends and another begins. One great feature is the ability to listen at 0.5x speed—slowing down a podcast makes it much easier to follow if you're a non-native speaker.
If you've been trying to get into Japanese podcasts, but just can't follow along without textual aid, Gengar Podcasts is your friend.