The parameters here are :
Authentication tag length = 128 bits (8 bytes)
According to the Web Crypto specification this must have one of the following values: 32, 64, 96, 104, 112, 120, or 128. The AES-GCM specification recommends that it should be 96, 104, 112, 120 or 128, although 32 or 64 bits may be acceptable in some applications: Appendix C of the specification provides additional guidance here.
IV length = 16 bytes
The AES-GCM specification recommends that the IV should be 96 bits long, and typically contains bits from a random number generator