GDPR has the force of law in its area of jurisdiction. If Slack can't comply with it, then they'd better not do business in that jurisdiction. That's how the law works; there isn't some hierarchy of one country's laws overriding others'.
Are you suggesting the US go to war with the EU to force them to repeal the GDPR, so that Slack can do business in the EU? I know HN typically takes a pro-business angle politically, but that seems beyond even the most rabid line that I usually see here.
If not, I am totally confused about how your response connects to what I wrote.
Other way around. The EU is the one who bears the burden of forcing American companies like slack to comply with their laws. Though jumping straight to a shooting war feels like an overreaction. Maybe start with a fine and ban the company if they don't pay.