gyno development is caused by high estro levels. letro inhibits the production of estro in the body. so yes, letro will prevent the build up of gyno, regardless what steroid is causing it.
deca alone shouldnt ever cause gyno, since it doesnt aromatize (convert to estrogen) very readily. when combined with other steroids, like test for example, it can cause a bit more of the test to convert into estro. either way, the letro would stop this.
i think your asking because of the internet hype concerning "prolactin" and deca. if i recall correctly, prolactin is what can cause a developed mammary gland (gyno) to start lactating. but it doesnt cause gyno to develop, that is estrogen.