I have written some code to do AES in GCM. I currently manually append the tag property to the ciphertext, is this the proper way to communicate the authentication tag?
I currently manually append the "tag" property to the ciphertext, is this the correct method of authentication?
That is a perfectly acceptable method. In the end, there needs to be some protocol which tells the other party where the ciphertext is and where the authentication tag is. In your case, the other party knows that the last 128 bits of the message contains the authentication tag.