I recently got a second hand Gaggia Classic and after testing it to make sure it worked, I descaled it with Durgol and flushed it one time. Now, two days later, I opened up the steam into a bowl and out came particles of dirt. So from there, I decided to flush it out some more through the steam arm and the group head. After 3 flushes, particles kept coming out. The previous owner told me that he backflushed it from time to time for maintenance but other than that, that is all he did. What could those particles be? What should I be doing now with it?

Edit: Upon taking apart the portafilter and steam wand, it looked extremely dirty and gross so I washed it off with water and scraped off any dirt I could get at. I should probably get some Cafiza to soak it in though.

You might take a magnet and see if it attracts the particles.