Can I open a second Gmail account in a separate app? I currently have several Gmail accounts, one for work, and the others personal. I'd like to keep these separate so I can forget about work on the weekends, but I'm finding it hard to do right now.

Is there a way to open the work account in a different app (E.G., Outlook) but not have it show up in the regular Gmail client as well?
Unfortunately the answer is that it will depend...

It will depend how your work email is setup and/or the device being used. 

Option1 - Android Enterprise
If your organization has an MDM (mobile device management) and they support a configuration called Android Enterprise it is possible they can distribute another instant if Gmail that keeps your work and personal email separate.  You will need to contact your companies help desk for more info.  Note your device must be new enough to support Android Enterprise. 

Option2 - Samsung Devices
Some newer Samsung devices allow you to do this with a feature called Secure folder.  Secure folder allows you to add applications like gmail to the secure folder.  This allows you to run another instance of gmail or most other android.

Hope that helps