I will go through the logical errors in this. Taking 'all's fair in war' and 'a gentleman does not act in secrecy' as examples, 'a gentleman does not act in secrecy' refers to being open and honest in interpersonal relationships. On the other hand, 'all's fair in war' involves using strategies in military competition. The contexts in which these two phrases are used are different. Comparing them here is an improper analogy in terms of logic.