Hi there! Nickel and Titanium coils must be run in TC mode; Stainless Steel can be. Kanthal and Nichrome must be run in wattage mode; Stainless Steel can be. Definitely do not run your Ni coil in wattage mode. Nickel can outgas some nasty stuff if it gets too hot. This isn't likely at vape temps-- you pretty much have to heat it until the wire melts in two-- but it is more likely in wattage mode than in TC mode.
There is a long, complex technical explanation of why some wire types are suitable for TC and others not, which I will be happy to explain to you if you are interested, but the bottom line on it is, while Nickel works in TC, when viewed from the point of sucking on the drip tip, Nickel doesn't work all that well. I jumped on the TC bandwagon when it first arrived and Nickel was the only choice. It was so terrible I gave up TC entirely for months and never tried it again until Stainless wires became available. Some people like it, I'm sure-- there's nothing so bad somebody somewhere won't love it-- but Nickel just doesn't do it for me and many others.
Your GS Air is not really designed for big cloud, it's more of a tootle puffer tank. It's lowest wattage coil is .75Ω, and while subohm, it's still pretty tall. If cloud is what you want, you will undoubtedly get more from the TFV4. Still worse, Ni is the only TC coil available on the GS, and it doesn't have a RBA deck so you could wind your own.
You do not mention which eLeaf mod you have. Let us know and we can try to get you sorted out regarding a different tank or whatever that will work better for you. With your present gear, the .75 Kanthal coil between 8-25 watts is probably your best bet for cloud on the GS tank.
If you want to run TC on your GS, try setting the watts at about 25-30, and the temp starting at 375ºF. The wattage is not all that important in TC. The mod treats it as a speed limit, a "do not exceed" value. It will use your set watts to get the wire up to the set temperature, then back off the power and use less watts to keep it there. You don't need to increase watts unless you are suffering bad ramp time, that is push the button and have to wait for vapor to be produced. With those settings, run the temp up from 375 until you get the vape where you want it. I usually vape in the 425-475ºF range, myself.
You say the vape gets hot. It definitely will running Ni in wattage mode, but it shouldn't in TC mode. Give it a try and let us know what's going on.
As to how to set up a TFV4, it will depend on what coil you have in the machine.