Gray propaganda is when someone tries to convince others to believe something without fully telling the truth. It's a bit like when you didn't really eat all your vegetables but you say you did because you want your mom to be proud of you. The person spreading gray propaganda doesn't really tell lies, but they don't tell the whole truth either.
For example, imagine I wanted to convince you that apples are the best fruit. If I said that apples are really healthy and delicious, that would be true. But if I didn't tell you about other fruits like bananas or strawberries that are also great, that would be gray propaganda.
Gray propaganda is like being a little sneaky with the truth so that people will believe what you want them to believe. It's not as bad as telling straight-up lies, but it's still not being completely honest. That's why it's important to ask questions and look at different sources of information so that you can make up your own mind about what to believe.