As a starting gun, the Z-1 Assault is useful for having good firepower, accuracy for rookie players. However, the Z-1 will become ineffective for later levels as it suffers poor stats when compared to other Assault Rifles. Also, It can't be augmented without EACs.
You should sell this gun not for money but a slot or simply replace it with a better gun that is randomly found in Strongboxes. Though the Z-1 Assault can be augmented with EACs, It's recommended not to do so due to EACs' rarity.
Z-1 Assault is modeled after the M4 (most likely the Colt Commando variation) with a futuristic design. It has the word "Z-1" right in front of its trigger. It has scratches & some duct tape wrapped over its stock, carry handle, grip and handguard, which proves this gun has been well used and is now in bad condition but still somewhat usable.
This is one of six weapons that have no or variants.
This is also one of three starter weapons, which is most likely why these weapons don't have or variants.
In game file of mobile version, there is an image of Z-1 Assault (mobile) that is black & gray (possibly through Black & White Constrast effect). That is image is unofficially claimed as BLACK version of Z-1.
The Z-1 assault is 1 of 8 weapons with their name visualized on them