The meanings of "マジネタ" and "ガセネタ"

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Question: What are the meanings of "マジネタ", "ガセネタ"?

These words are a composition of three words:
マジ = truth, real, earnest, serious
ガセ = false, fake, lie, unreliable, inaccurate
ネタ = information, story, subject, topic, material, stuff

All of them have various meanings. "マジ" is a word opposite to "ガセ".

The basic meaning of "マジ" is truth, "ガセ" is false. So "マジネタ" and "ガセネタ" can be used as follows:

A: おい知ってるか？　明日ここに大統領が来るんだって！
    Do you know? I heard that president Obama will come here tomorrow!
B: まさか。そんなのガセネタだろ。
    No way. No kidding!
A: いやマジ、マジネタだってば！
    Believe me, I'm not kidding!

The Original meaning of "ネタ" is material, stuff.

"ネタ" means materials that should be prepared for some activity.
For example, a chef need to prepare the ingredients before he do cooking.
In this situation, ingredients are "ネタ" for dishes.
So Japanese sushi-chefs called fishes as "ネタ" in the old days, Edo period.

Later in the middle of the last century, comedians began to use the word "ネタ" for the meaning of "a topic of a joke". Comedians need to organize "a topic of a joke" before they perform it on their stage. So they considered "a topic of a joke" as fishes for sushi.

And later that, the word "ネタ" gained a new meaning of "a core topic of a story".

At present, the word "ガセネタ" is very popular in Japan. Used as a slang of "false story".
("ガセ" means false)

In comparison to the popularity of "ガセネタ", "マジネタ" is not widely used, but its meaning is clear to all Japanese. You can use the word as the meaning of "true story".
But be careful, "マジネタ" is a slang. Don't use it when you are making a ceremonial speech or like that.