I haven’t done such inspections myself, but if I had to select the GSD I’d check how big those leaks or infiltrations usually are. Then, I’d take a GSD that is at least half that size, so that you can detect the leaks.
Here is a similar post on the subject: https://community.pix4d.com/t/4020
As for the radiometric vs non radiometric camera, one advantage of the radiometric camera is that you will have absolute temperatures, so you could search for leaks by searching temperatures directly. The other would be more approximative and maybe more a visual inspection. Make sure to create the reflectance map in the software to make this analysis.
Hope it helps!
Would be great to have some people doing thermal inspections commenting! 