yes, why is there suffering on earth

That's one of the oldest and heaviest questions, and I'm glad you brought it here. What made you ask it? Is it something you're seeing around you, or something you're going through yourself? One verse I keep coming back to: "The One who created death and life for the purpose of distinguishing those among you who would do better. He is the Almighty, the Forgiving." (67:2) To me it says life isn't random. It has a purpose, and GOD is not careless about us in it.

what does the bhagvad gita say about GOD and life suffering

Good question. I'm not a Gita scholar, so please check with someone who knows it well. But as I understand it, it speaks of the eternal soul, doing your duty without clinging to results, and turning to the Divine to find peace through life's pain. I really respect that search for steadiness. For me the common ground is this: "We have sent a messenger to every community, saying, "You shall worship GOD, and avoid idolatry."" (16:36) So I feel the call to turn to the One GOD is very old and very wide. What drew you to the Gita? Is there a part of it that spoke to your own questions?

was krishna a demi god in gita

Honestly, I'm not the best person to answer this, so please check with someone who knows the Gita well. As I understand it, most Hindu traditions don't call Krishna a "demigod" in the sense of the lesser devas. Many see him as an avatar of Vishnu, speaking in the Gita as the Supreme, while others see him more as a divine teacher. So readers differ. From the Quran I can only say this: "We have sent messengers before you - some of them we mentioned to you, and some we did not mention to you." (40:78) So I can't say who Krishna was. But I hold that GOD is One and none equals Him. What did you feel when you read his words in the Gita?

how can you state for sure that the Quran is from GOD

Fair question, and you shouldn't take my word for it. The Quran itself says: "You shall not accept any information, unless you verify it for yourself. I have given you the hearing, the eyesight, and the brain, and you are responsible for using them." (17:36) So I'd say it's belief resting on evidence you can check. One example is the 19-based code (see 74:30). The first verse, the Basmalah, has 19 letters, and the Quran has 114 suras, which is 19 x 6. I might be wrong about how convincing it is for you, though. What would count as real proof for you?

can you tell me what the Puranas say about GOD, was there a messenger involved