The word gossip is sort of like the word tea. You don't "drink a tea", you "drink some tea", or just "drink tea". You don't "hear a gossip", you "hear some gossip" or just "hear gossip". I would probably use the word rumor instead. For example, "There was a rumor that ___, and I just learned that it was true."

You typically wouldn't say "there was a gossip", as gossip isn't used with the indefinite article "a". Rather, it would be more natural to say "I heard some gossip and just learned that it was true". Your response is grammatically correct but feels slightly stilted. If the gossip was relating to a person, it would be better to say "It's true what was said about him/her". If you want to just reference the gossip in general, then you can simply say "What was said is true."