The BCM4778 was announced on 22 Sep 2021 in Broadcom’s product news and GPSWorld. It is the third generation dual-frequency GNSS receiver from Broadcom, measuring just 2.4 mm x 2.7 mm x 0.65 mm.
The BCM4778 is listed amongst the mass-market premium family of GNSS receivers in the GNSS and Secure SATCOM User Technology Report 2025 / issue 1, published by EUSPA. See page 39.
The BCM4778 is still listed as active in the list of GNSS/GPS SoCs on the Broadcom website. Further information can be found on the product page and in the product brief (PDF).
Synaptics also produce the BCM4778 chipset, but it is marketed as the SYN4778. The Synaptics chipset is used in the Garmin fenix 8, clearly shown in the teardown by F Tipi in April 2025.
I suspect the BCM4778 is used in Apple watches with an S9 or S10 CPU: