Reply from same address as mail was sent to in Mail.app I have a single Gmail account that receives mails via forwards from 2 other addresses. The Gmail web UI allows me to reply with the address it was received on.

Is it possible to set up Apple's Mail.app to automatically use the recipient as the sender address in replies (without having multiple distinct Gmail accounts)?

I've set up aliases on the specific account, but Mail doesn't seem to automatically choose the correct one.
Apple Mail can only send replies from mail enabled accounts that you have set up in your system. I.e., you may have iCloud account set up in your system, but if you have mail option unchecked in System Preferences -> Internet Accounts -> iCloud, you won't be able to send any mail (including replies) from that iCloud account.
One possible workaround is to use 'Reply To' field which you can enable by pressing Alt+Command+R or from Main Menu -> View -> Reply-To Address Field. This will enable your recipients press reply button and send their reply to the address which you will enter into mentioned field.
Hope this helps.