They all used "coerced" labor, from the occupied territories, though the treatment varied with the firms.
Steyr used slave labor primarily on construction projects early on (1942 on) and the parts in their firearms (98k) were made primarily from a mix of slave labor (Jews) and less than enthusiastic Poles at their Radom & Warsaw factories 1940-41 on.. most of the Jews were sent to Auschwitz on or about 20 July 1944 (about 1800 Jews altogether). With the loss of Radom, KZ Gusen was also given the task of making 98k components; they had only been involved with MP component manufacture up to that time. No 98k was assembled at Steyr after 1942 anyway, as it was moved to make room for more important aircraft related production.
Gustloff Weimar used slave labor also, but not exclusively, they had a lot of German workers, and coerced workers, at their actual factory (not to be confused with their operations at Buchenwald)- rifles (98k) were but a sideline for them, very unimportant to the overall structure of the company.
Although the nazis-ss increasingly forced slave & coerced labor on the industrialist, the treatment and conditions varied widely depending on the company and men operating the plants. Slave labor conditions were much worse in all cases as generally the ss had more of a role to play when they were used, but actual “slave labor” was only given to firms that had "special" ties to the party or government. Steyr & Gustloff were government owned firms, so they had access to funds and labor most privately owned firms couldn't acquire. (When I say "privately owned" I mean that is the looses possible way- the owners controlled very little of how their property was managed or operated, from raw materials to labor to profit taking the government had the strictest controls, and you made what you were allowed to make generally. Often even "free German labor" was moved around without a firm, or the worker in most cases, having a say about it- even top nazis like Milch couldn't stop labor plundering... even Speer lost control to the ss near the end.)
Regardless of labor source, manufacturing was strictly controlled and the penalty for sabotage, work slow down, or any hinky business was severe and “sabotage” in the traditional sense is very overstated. Generally workers worked to survive, the Czechs postwar reports state it best- they did what they had to but nothing more, it was dangerous to try to resist openly, and even if you could get someone replaced (a manager you didn’t like by inefficiency or poor work) you would often just get a worse manager and even more scrutiny. The real problems of latewar rifles has more to do with short cuts and raw material issues, but any rifle with a fireproof and final acceptance should be relatively safe to shoot, with the proper ammunition and normal safety practices. (even late in the war quality control caught bad batches of barrels before leaving the factory – Döhlen barrels specifically)