If I set up Apple Mail, will it delete my emails?
The .eml file is a mail file that is based on the MIME RFC 822 specification.  It's basically a text format that pretty much all Mail applications can open

Outlook (macOS, Windows, iOS, Android) / Outlook Express (Windows Only)
Microsoft Mail (Windows 10)
Apple Mail (macOS)
Mozilla Thunderbird

Regarding your question, you don't need to setup a Gmail acccount (or any account for that matter) to read a single .eml file.  If it asks you to set one up, just skip it.
You can also open a .eml in your browser.  Simply rename the file with an .mht extension.  Assuming the file is on your Desktop, using Terminal
% mv ~/Desktop/foobar.eml ~/Desktop/foobar.mht

Then open with your browser.  If it's only text, any browser will work, but if it has embedded images, Chrome or Microsoft Edge will natively display it (I prefer Edge over Chrome as it uses the same engine, but I distrust Microsoft less than I do Google).
Microsoft Word will natively open both a .eml and the .mht.  If you don't have Word, you can open it with the free web version of Word.  You just need to create a free Microsoft account.
As an example, I got an old .eml file (an Uber receipt) and tested.  It opens natively in

Outlook for macOS
Apple Mail
Microsoft Word
Edge/Chrome (as a text file)