Can we get the gas compositions from a gas planets? Is it possible to get some gases like hydrogen from gas planets (Saturn, Jupiter,  Uranus or Neptune)?
Yes.  It's certainly feasible to obtain a sample from Jupiter or Saturn, effectively getting gas from the planet, but it's not easy.

Massive planets, as a byproduct of their high mass, create large gravity wells which require a lot of fuel to fly close enough to get a sample from and return with it to where it's needed.  It would take perhaps a ton of conventional fuel to return from Jupiter with a few pounds of gas, which isn't a good return on investment.

Collecting hydrogen from Jupiter would be expensive almost certainly not worth the cost.   It would be much easier to obtain hydrogen in space from frozen water ice, frozen methane or frozen ammonia, where hydrogen is abundant.     This can be obtained in icy comets and there's even some water-ice on the moon in craters, which has a much smaller gravity well.   Also on Mars' ice caps and inside Ceres, it's believed there's about an entire oceans worth of water, perhaps more water inside Ceres than on Earth.  

That's a lot of hydrogen that would be far easier to harvest than hydrogen from Jupiter.   And while it would need to be separated from Oxygen, chemical separation would require many times less energy than collecting hydrogen from a gas planet's gravity well.   You'd also have the advantage of not just harvesting hydrogen but oxygen as well.

It's not impossible to harvest hydrogen from Jupiter or Saturn, it's just difficult to imagine it being worth the trouble.