On a new installation running systemd and GNOME, emerging net-im/slack with the ~amd64 keyword pulls down version 4.4.3 of the package; attempting to launch this application results in it crashing. Attempting to launch it from the command line reveals more helpful information: a message that protests that /opt/slack/chrome-sandbox should be owned by root and have mode 4755, and that the application will now crash because these things aren't true. Manually resolving those permissions allows Slack to be launched normally. Presumably, this step should be incorporated in the ebuild unless there is good reason for it not to be. An earlier issue (Bug ID 713094) claims to have fixed this issue; however, as of today's date I can verify that it is still present.
Did you emerge it with USE flag suid?
Emerging slack with suid USE flag should result with /opt/slack/chrome-sandbox has permissions 4755.
Without suid USE flag it will have it set to 755.
In both cases it will be owned by root:root.
Please, emerge it with suid USE flag and provide
ls -la /opt/slack/chrome-sandbox