Considering the population who uses this drugs, total noobs, I seriously wouldn't discount the fact that gyno was preexisting and that they just didn't know about it.
I would suggest looking at pre androgen use pictures to see if there is anything there.
And I would also suggest ordering only from sources that lab test their every batch and that also have random lab tests from users. That should just be a fixed rule of thumb for everybody in the game of aas usage. There is absolutely no need in ordering from any source that doesn't do this, no matter what is it that you are ordering.
And again, getting gyno while on 20mg of tamox is really hardly possible. Maybe when lowered from 20 to 10 some sensitivity could happen if estrogen was high and free androgen index was really low. But even that shouldn't happen immediately, as tamox has a long half life and besides; it takes some time for an actual hard lump to grow. You first get "puffy nipples", and even that doesn't happen right away. All of this is, again, the reason why I'm saying that there is a big possibility that gyno was preexisting.