Should be proof enough that it came from a different source than you usually charge your account with? Just make sure to collect all evidence and guard yourself for future issues.
Gmail:
bottom right hand corner "details", shows all recent login sessions
Use 2 factor authentication https://www.google.com/landing/2step/
Steam:
Top right hand corner, click your wallet amount, check your recent transactions
Enable steam guard (2 factor authentication)
Go to steam > settings and then family, see if there is an eligible account you don't know. (just in case)
Bank:
If you want to be diligent, you can place a fraud alert (procedure depends on your bank, inform with their customer service.
Both Steam and Google have authenticators for the phone, so if your phone isn't compromised you should be able to guard yourself a bit better than usual.
The way I know steam account hijacks work is that (often) Russian ATM's have an option to deposit money on a steam account. If they know the username they can use the statement to show they're the owner of the account, and transferring account details to their possession.
Hope you can work it out, stay safe buddy!