A 'tacit understanding' or 'tacit agreement' can be the gender neutral in many instances wherein 'gentleman's agreement' is used, but indeed I don't think the former can denote all that the latter connotes.

But what do you think it connotes? The meaning of "gentleman's agreement" that I grew up with is a tacit agreement of mutual back-scratching, generally to the detriment of those outside the club of insiders. It is not publicly enforceable because it is not publicly acknowledgable, but those on the inside will know when someone defects and discreetly edge them out of the club. It is not to do with honour and pride, in the hurrah senses of those words, but the old boys network.

The Wikipedia article on the subject defines it pretty much along those lines, and gives some historical background I wasn't aware of.