The texture fillrate calculator works out the texture fillrate based on the clock speed and texture mapping units.
Texture fillrate is calculated by the equation Texture fillrate = TMUs (texture mapping units) * Clock Speed (base or boost)
This gives the fillrate in MTexels/s so you can divide by 1000 to get it in GTexels/s
Usually the boost clock is used for texture fillrate calculations in GPUs.
View the Pixel Fillrate calculator here.
The Nvidia GeForce RTX 4080 has a boost clock speed of 2505 MHz and 304 texture units. Therefore, the texture fillrate is given by 2505 * 304 = 761,520 MegaTexel/s
To get this in GigaTexel/s divide by 1000 to give 761.52 GigaTexel/s