Apple is letting you mass-transfer photos and video stored on iCloud to Google Photos, it revealed in a support document earlier this week. You can find the service on Apple's privacy site under "Transfer a copy of your data," as previously reported by MacRumors.
You'll need to have two-factor authentication set up on your Apple ID and make sure your Google account has enough storage available. The process takes between three and seven days, which seems slow but still better than transferring everything manually.
If you've been tweaking photos to get them just right, only the most recent edit will transfer. Supported formats included .jpg, .png, .webp, .gif, some RAW files, .mpg, .mod, .mmv, .tod, .wmv, .asf, .avi, .divx, .mov, .m4v, .3gp, .3g2, .mp4, .m2t, .m2ts, .mts and .mkv.