If it is possible to siphon off 1/2 of JUPITER's gas, would that reduce the pull of gravity by 1/2 or not?

If Jupiter is made up of 100% gas, and you remove half of that gas, the mass and force of its gravity will be halved. It doesn't matter that the gasses are floating around. Technically, the Earth's atmosphere contributes to it's gravitational pull on distant objects like the moon. If you are on the surface, there is (very very small) pull from the atmosphere above you, and from the atmosphere on the other side of the earth, so they mostly cancel each other. From an external point of view, all the gases or solids just contribute to the total mass.

I'm not sure if Jupiter has a surface, but I suppose if it did and you stood on that surface and removed all the gas above you, the gravitational force on you would not change.

Jupiter does have gravity which is more than 2 & 1/2 that of Earth at it's so called "surface".. The gravity of Jupiter is what has allowed it to spawn at least 63 moons and many other objects and also to shield Earth from countless asteroids that are sucked in. But no, it doesn't have a solid surface so you would fall down more than 2 and 1/2 times faster than you would fall on earth towards the core and eventually be crushed by pressure similar to the way you would be crushed at the bottom of the ocean, except faster and gas pressure would be doing the crushing...

We still don't actually know why things have gravity but you should read about the insights that Newton, Kepler and Einstein made. A lot has been discovered about what gravity is or is not.... The quick version of it is that it is not actually a force but actually a cause and effect of movement and the absence of space and time the movement creates so things are kind pulled in along with other. Einstein predicted gravity could even bend and speed up the photons that make up light as validated in black holes. Very compelling work.

...And also yes in theory it's possible to syphon all the gas on Jupiter and some people predict that one day humans or our cyborg descendants will be doing this if we continue to advance our technology and need for energy rich gasses and of course this will have an effect on its gravity but I doubt it will actually change by 1/2 because the gravity system includes all the moons just like how our moon effects the gravity on earth... Its a complete system and not just an objects mass which spawns gravity... If it did Jupiter's gravity would be 160 times that of Earth.