If people making stuff for people to buy and use can be found culpable for one reason or another
,
of doing harm to themselves or others.
can the gun manufacturers be far behind?

or has this been decided already in the courts?

It has been tried a number of times and generally been unsuccessful. I believe there was litigation after Sandy Hook, but I am not sure. The issues with opiates and Roundup are somewhat different in my opinion, as there seemed to be more of a nexus.

Probably apples to oranges. I think at least some of the cases against stuff like prescription opioids and Roundup is that the manufacturers knew of dangers/abuses but they were intentionally trying to hide that info from the public, knowing harm would come from their deception. With guns, the manufactures won't deny, if you shoot someone with their gun, it's likely to do harm.

Search for PLCAA.
Protection of Lawful Commerce in Arms Act