In Breaking Bad, Walter needed money to support his family at any cost, however, how would a highly intelligent and independent man like Gale Boetticher end up working for Gus Fring? I know it was a question of exercising "chemistry" for Gale but was there anything else in it for him?
I recall Gale explaining to Walter:
I'm definitely a libertarian. Consenting adults want what they want and if I'm not supplying it they will get it somewhere else. At least with me they're getting exactly what they pay for; no added toxins or impurities.
I think it's also useful to consider the purity of meth. he could make, which was probably matched by Jessie's cook (using Walter's formula) on a good day, and only beaten by Walter's product. He was justifiably proud of the quality he could achieve.
That, and his love of the pure chemistry, was pretty much all the reasons I recall being offered for his decision to cook for Gus.
..it just makes no sense to me that he chose to exercise " true chemistry" in a form of blue meth rather than some useful medical application
I imagine the number of chemists that get to work on 'useful medical applications' is probably 1%. And of that 1%, 99% of them would be doing something mundane like ..testing the pH level in samples - 200 times a day.