It does not make gains more "retainable", it just doesn't give you that illusion that water weight gives you when you think you've gained 40lbs and half of that is water which goes away in a week after going off.
But as discussed a million times before over here, estrogen is also anabolic and essential for your health, so stay the fuck away from letrozole during cycle ... If you are using heavily aromatising substances you can use a little anastrozole, but that should be enough. 0,5mg EOD should suffice for any kind of cycle, if it does not, you need to lower your dosages.
IA