The information provided by the gunslinger TV series did not mention that the series had 40 episodes, nor did it mention that it could be watched for free. Therefore, I don't know if the gunslinger TV series has 40 episodes and if it's free to watch.
Well, an omnibus in the world of comics is basically a single book that brings together a bunch of separate comic issues or volumes. It makes it convenient for readers to have a chunk of related content in one place. It can save you from having to hunt down individual issues.
A manga omnibus is basically a collection of several volumes of a manga series in one book. It's convenient if you want to have multiple parts of the story in one place.