The same terraforming techniques used in the Red/Blue/Green Mars series wouldn't work on a gas giant. But some form of terrafroming may. The thing about gas giants is that their atmospheres are layered and of varying composition. Their cores are so high pressure that they are essentially molten. We'd never be able to live down there. However, many gas giants probably have a layer in their atmosphere that would be the right pressure and temperature for human habitation. It may not have the right atmosphere composition.
Most gas giants have a primarily hydrogen/helium atmosphere. Mars' atmosphere is primarily CO2. So in the Red, Blue, Green Mars series, what they are doing is causing runaway global warming (releasing more stored CO2) to increase atmospheric pressure and surface temperature. Then they are planting plants to convert the CO2 to O2.
It's probably not possible to use similar atmospheric modification techniques to adjust the composition of gas giants. It depends heavily on the composition of the atmosphere at the correct temperature and pressure layer. There are trace amounts of CO2 and Oxygen. 778
And since the layers stratify, it's possible that there could be an oxygen layer or a CO2 layer. They may or may not line up with the correct pressure and temp. If an oxygen layer does, we're in good shape -- no terraforming required. If it doesn't or a CO2 layer does, then its likely that it would not be modifiable, because any oxygen created would rise or fall to a different layer. If, by some strange coincidence the oxygen layer is only slightly above or below the CO2 layer, that is the one circumstance where similar techniques could work. Grow the O2 layer and it will take up the space (maybe) vacated by the CO2. On the otherhand, if the correct layer is helium or hydrogen, then the techniques used on Mars would not work. Period. Also, this would require some sort of technology that would allow us to float platforms or airships pretty much indefinitely with out too much energy expenditure at the correct level in the atmosphere.
In Venus' case, the problem is the reverse of that on Mars. On Mars, the terraforming involves causing runaway global warming -- basically. On Venus, the problem is reversing it. So it would require completely different techniques. 778