For some reason, it reminds me of the old "got milk?" commercials.
I also looked it up. It is indeed marijuana...and a data processing system or something. Or "bad, dirty, or generally unpleasant". Either way, I don't want to know what the advertisement was supposed to mean.
Urban Dictionary says "grut" relates to Marijuana, but I can't imagine why American slang would appear in a French cartoon, and somehow mean the same thing.
My own research (mainly Wikipedia, I'll admit ; ) says that "grut" (also called "gruit" ) is an herbal mixture used for flavoring beer so perhaps that is a beer/liquor ad (which, depending on what the target age group for that magazine is that Sissi's reading, would probably make sense).
Grut also seems to mean something akin to "great" in one of the Germanic languages, but I can't find hard evidence to support this.
"Hey, make up your mind. Am I a genius or a creep?"
"You're a creepy genius."