Hi @dwest,
Our Slack import is still in Beta, and that's one of the issues we still need to fix (being tracked in Jira). The solution being considered is to let the administrator select an option when these collisions happen - for example, to append a number to the username, change the username, merge, or cancel/skip the user. Do you have any feedback on what would work best for you?
In terms of a work around - are you just looking to get the history imported from Slack, if people already have accounts authenticating with GitLab? One option might be to ask the user to change their username in Slack to be slightly different, so you can export the history + import it to Mattermost without the username collision.