These are the things I would like to know.

-If an attack scores a critical hit in G/S are the Att, Def, Satt, Sdef, stats are based on the original stats as it was in R/B/Y?

-Are multi turn attacks damage calculated like this: The attack hits in the first round with the damage calculated from the attacks base damage,than it damages 1/16 max hp in turn 2, 3, 4, 5?

-Can somebody explain to me how are the reversal, magnitude and flail attacks are working?

-What are the effects of the badges in the R/B/Y/G/S?

-What are the effects of Dire hit, Guard spec. and happiness egg In the G/S versions?

-What if Gurd spec. Is used with an attack which halves the damage of Special based attacks?

I hope that someone can explain these things to me.

Are multi turn attacks damage calculated like this: The attack hits in the first round with the damage calculated from the attacks base damage,than it damages 1/16 max hp in turn 2, 3, 4, 5?
Only for Bind, Wrap, Fire Spin, Clamp, and Whirlpool. Note that the other pokémon cannot switch unless by Baton Pass, but can still attack.

Can somebody explain to me how are the reversal, magnitude and flail attacks are working?
Magnitude is a calculated attack that does either 10, 30, 50, 70, 90, 110, or 150 base damage, which can still be modified by evade/accuracy and the randomness factor that's in the BDF. It also does double damage to pokémon charging up on Dig.
Reversal and Flail are the same: they do more damage the lower your own HP is, from 20 power to 200 power. Reversal is a Fighting attack, while Flail is Normal.

What are the effects of the badges in the R/B/Y/G/S?
Pass...

What are the effects of Dire hit, Guard spec. and happiness egg In the G/S versions?
Dire Hit: works like Focus Energy
Guard Spec: works like Amnesia (in contrast to X Special, which works like Growth)
Lucky Egg: I don't know, but Lucky Punch raises the CH for Chansey...

What if Gurd spec. Is used with an attack which halves the damage of Special based attacks?
It's either:
the two are independent from each other, or
it goes from 2/4 to 2/5.

I'm still not sure about the Critical Hits in Gold/Silver penetrating stat-modifications or not. The latest that I have heard is that they WILL penetrate like before if it is to the attacker's advantage. But if it is to the attacker's advantage to just keep all stat-modifications as normal (for example if the attacker has lots of Swords Dance and the defender just has Reflect up) then that is what the game will do. In other words...the game always does what is best for the attacker that got the CH. I have just heard this...I do not know if it is true or not.