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A gentleman shouldn't fuss over trivial matters.
Literal
Gentlemen [subject-が] small matters [to-に] particular not [copula-ではありません].
殿方 again. 細かいことにこだわる ('to fixate on small/detailed things') combines こだわる ('to be particular about, to obsess over') with 細かいこと ('small matters'). ものではありません is the mild prohibition pattern — 'one should not.' The sentence delivers an old-fashioned gender-norm proverb with formal grace.