I have my aircraft set up with a Garmin inReach for ACAS and SE-SAR on 1800wxbrief.com. If I file using the Garmin Pilot app, will the inReach ACAS and SE-SAR work or only if I file on the 1800wxbrief.com site?
Typically, SE-SAR and (SMS-based) ACAS are only available for flight plans that are either filed directly through 1800wxbrief.com or through an app that's linked to your Leidos account (1800wxbrief.com > Account > Service Provider Authorization). Garmin Pilot isn't on that list, so flight service won't be able to match your flight plan with your Leidos online profile... meaning these services won't be activated.
Some apps, like Foreflight, have their own built-in ACAS feed, separate from 1800wxbreif.com's SMS-based interface. If your app has that, it would receive ACAS directly. However, I don't believe Garmin Pilot has this feature.
That said... Garmin recently acquired FltPlan.com, which can integrate with Leidos accounts. There is a way to link Garmin Pilot to your FltPlan.com account for syncing data (instructions here), but it's not clear which system's backend is actually used for filing. I'd recommend giving it a try, it might work. (I don't have a Garmin Pilot account to test.)