Why? Is it 2 hot 4 u?
The GDDR6(X) critical thermal limit is in range of 105-110°C, and if it stays below 90°C you're all set and dandy.
Thermal pad replacement is valid for RTX 3080/Ti and 3090 which has a "fiery temperament", but normal GDDR6 as in Radeon 6000 series and RTX 3070 and below does not overheat under normal conditions. If yours does I would recommend contacting support services for a warranty case.
If you still insist for personal (overclocking) purposes, I can only extrapolate the info on RTX Founder's, and say that it would take a 1.5 mm thermal pad, or soft and squishy 2 mm, but that's a theoretical assumption and if you're scoring a non-reference design it may be anything at all from 0.5 to 3 mm.
