This is a specification that isn’t always included in the datasheet, in the case of Bergquist/Henkel’s TGP 3004SF series we had to get clarification from them directly. Here is the specification they provided:
“The recommended compression range for our Bergquist Gap Pads is 15-40%. Though we do not have thermal impedance data available for the TGP 3004SF, the value should remain steady if it is compressed within this range.”