I regularly have emails to my GMail from colleagues end up in the spam folder. Perhaps as many as 5 or 10 per week, enough that I have to trawl through the spam folder every few days to make sure I've not missed something important.
Similarly, reasonably often when people reply to me, often after being prompted with a second email, or even a phone call, people tell me that emails from my "other" ISP to their GMail provided mailboxes have ended up in the spam folder.
It happens in waves ... it can be fine for months, and I start to think it's all good and I can trust it, and then there's a spate of emails going to spam when they really shouldn't. It's simply unreliable.
So for incoming email I do my own filtering. I can do nothing about my outgoing emails ending up in someone else's GMail spam folder except to be aware that it happens, and to keep track of when I'm expecting a reply. With some colleagues I've gone to a "positive acknowledge" system to preempt what have been expensive problems in the past.