Sometimes when I gun it on my gs350, i feel a jolt and hear a clunk.
Is that stability control?

 

if you are trying to do a burnout from a stop (or very low speed), that's probably not the VSC (stability control) but the regular traction control, which cuts back engine power, applies the brakes to the drive wheels (or both) to stop them from spinning. When regular traction from the rubber tires is again made and the spinning stops, the system then cuts out.

Here's one way to check: Next time you feel the jolt, look at the dash and see if a yellow or orange-colored light is on or goes out. That will tell you (if the dash-bulb itself is not burned out) if the system is working properly and actually engaging. Both Traction and Stability-Control dash lights usually look something like this:

 

In my 2IS and my moms 2RX, Stabilitly Control is audible when it kicks in usually on a hard turn. Sounds like multiple beeps when activated

 

Jolt and clunk are not the best descriptive words to help pinpoint what's going on with your vehicle. Is it a new GS350? previous gen? etc. Need more details. As Hoovey said, there's generally an audible cue when things kick in for Lexus vehicles.

 

it must be traction control. The clunk must be when tire gains traction again.

 

simply: yes.

Just remember that you got saved by stability control so next time try to be as fast as possible without it activating.