I'd say, Garrisons are neither PC markers nor Units - they are Garrisons. :)
Thats right - but in some places they are described as "not combat unit"; in some others rulebook claims, that they have functions of PC markers.
So even if some of us have serious doubts about that, we are removing garrisons along with PC markers during such events. Thats why I finally decided to seek an answer here - whether I like answer or not ;) this will finally spare some time wasted on discussions every time we are playing :)