GMeet is a product tailored for business customers of Google, who often use Google Hangouts as a way to hold video calls between different employees. But Google Hangouts isn't tailored to business users.
Details on what GMeet can actually do are scarce right now. All that has emerged is a pair of screenshots. But it seems likely that it will include features aimed at business "enterprise" customers that aren't found in Google Hangouts.
Right now, only Google employees are using GMeet, but it could be released to new users in the future. Interestingly, a code snippet for the app seen on this blog post shows that it might have been in existence since at least 2011.