вы можете сделать так:
private func setupMyLocationButton() {
let locationButton = mapView.subviews
.filter { $0.description.contains("GMSUISettingsPaddingView") }
.flatMap { $0.subviews }
.flatMap { $0.subviews }
.filter { $0.description.contains("GMSx_QTMButton") }
.first
let customImage = UIImage(imageLiteralResourceName: "yourCustomIconFilename")
let myLocationButton = locationButton as? UIButton
myLocationButton?.setImage(customImage, for: .normal)
}