No, you’ll continue to send/receive email as usual. To use Gmail with Google Workspace, you setup your domain (MX records) to direct your mail flow to the Google mail servers. It’s like registering your new address with the post office so that mail is delivered to your new location. Once the MX records change takes effect (up to 72 hours), you can start receiving/sending email through your Google Workspace account. Until then, you’ll receive/send your email through your old mail system.