I'm not sure how that could even be an issue. You need access to the gmail account to confirm the email either way. The only possible issue I can imagine is a poorly built site assuming that all email servers do this and ignoring dots in emails.
Assuming one John Smith creates an account as john.smith@gmail.com ; and then another John Smith happens to create an account as johnsmith@gmail.com - and doesn't confirm the email right away (either the site doesn't ask for it; or doesn't really block any functionality - just keeps showing an annoying warning message). First John Smith can now easily take over the second account by triggering a forgot password workflow - for shit and giggles.