If you are a company that provides spamming capabilities to anyone and everyone, then it is exactly valid to add them to a spam list.
You could say, "Plenty of legitimate users use gmail." Fine, maybe so, but if you are someone who uses gmail, you are surrounded by a cadre of spammers and should understand your credibility is instantly devalued.
Birds of a feather flock together. The solution is to stop using gmail.com.
A VERY VERY high percentage of spam in my inbox comes from gmail accounts. If tens of percentages points of spam is sent from a domain, then it's valid to add them to the list of spamming domains.