Looking for Filter Gallery

Check also what is selected in the Layers panel. Filter Gallery is active only when filters can be used. If you have curves thumb selected, for example, Filter Gallery will be greyed out. Select pixel based layer then try. Also see explanation above.

Is your Photoshop document in CMYK? If it's in RGB, is it in 16 or 32 bit? The filter gallery is disabled in these modes. Put the document in RGB 8 bit to enable it.
https://community.adobe.com/t5/photoshop-ecosystem-discussions/filter-gallery-greyed-out-why-and-how-do-i-get-it-to-work/td-p/9810880

if it's greyed out, then something is not right with your image. Are you working in 8-bit RGB mode?