I'm glad to see that you got it working properly. That's great. The <200ns rise/fall times aren't super critical(in some cases), it's just the goal I like to have in my power supplies to minimize heating in the switching devices. My power supplies are usually low-voltage(10 to 100VDC) but high-current(10 to 100Amps and I'm working on perfecting my design and increasing the current). I also like to push things to the failure point to ensure that my supplies will be reliable when operated under normal conditions. The faster my switching times are, the less heat is generated in my switching devices, and therefore my supplies can then withstand a longer duty-cycle before the switches overheat. I would imagine in a lower-power supply that having super-fast switching times would be less important just so long as there is no shoot-through and they are fast enough that the switches don't spend a ton of time in their linear regions.
Now, having said that. I reread some of your older posts and realized that you said you were driving your GDT directly from the outputs of the SG chip. Now, according to the datasheet these outputs can source/sink a maximum of 500mA but their recommended operating range is around 200mA. I also just noticed that the "typical" rise-time of the outputs is 100ns but can be up to 600ns. The "typical" fall-time is 50ns but can go all the way up to 300ns. Since you're driving the GDT directly from the SG chip, your GDT cannot provide a drive current to the FETs that is greater than the drive current going into the primary side of the GDT. Actually due to some transformer losses, the FET drive will be slightly less than the primary drive current. Too little drive current to the primary side of your GDT can also cause the slower rise-times and sloping tops. What happens is, when you increase the inductance of the GDT to minimize the sloping tops, the rise/fall times will suffer due to the greater inductance. If you have a way of buffering the outputs of the SG chip to provide a greater drive current(ie: Transistor Totem Pole) to the primary of the GDT you can then balance your rise/fall times and the voltage droop at the peaks by adjusting the inductance of the GDT. If you don't have enough drive current going into the GDT, achieving flat tops on your waveform will require way more inductance than is really necessary and your rise/fall times will suffer significantly. As you increase the drive current for the GDT, you can lessen the inductance of the GDT, improving the switching times, while still maintaining minimal voltage drop at the peaks.
