Psychological inoculation interventions, which seek to pre-emptively build resistance against unwanted persuasion attempts, have shown promise in reducing susceptibility to misinformation. However, as many people receive news from popular, mainstream ingroup sources (e.g., a left-wing person consuming left-wing media) which may host misleading or false content, and as ingroup sources may be more persuasive, the impact of source effects on inoculation interventions demands attention. In this experiment, we find that although news consumers are more susceptible to (non-political) misinformation from political ingroup publishers pre-intervention, gamified inoculation successfully improves veracity discernment and reduces susceptibility to misinformation from both political ingroup and outgroup publishers.