How to identify part of line in txt and than take entire line

I've bumped on next problem:
I have txt file with bunch of mails, and I want to write script that search for specific one and put it in other txt file.

Example:

Email1@gmail.com
Email2@gmail.com
Email3@yahoo.com
Email4@hotmail.com

Now what I want is to search if domain is @gmail and output entire mail in new file.

I have tried StringInString but I have no idea what to do with number of place where is found line starting.
I'm pretty sure this should be done with StringCompare but I seams not to be able to find anyone with similar task who have posted online about this. I would appreciate any help or idea you could have and could share with me. Thanks