According to the news articles I have seen Lego have sent a cease and desist letter, however I suspect that this is more a public relations exercise by Lego than any serious attempt to get the manufacturers of the gun to cease production and I suspect they will just ignore it.
Lego have already lost in court several years ago to prevent the production of Lego compatible building block (see Lego Vs Tyco) so as long as the firearm doesn’t reproduce the Lego logo (which is still a valid trademark) on the top of every bump as Lego do on their actual blocks the chances they can stop the production of a brightly coloured gun with a regular pattern of bumps on it is pretty low given they couldn’t stop the production of an actual building block with Lego compatible dimensions.