This little Go application will copy everything from one Google Cloud Storage Bucket to a MinIO Bucket. You'll need a GCS credentials file and a MinIO access key + secret.

copy_gcs_bucket_to_minio.go

package main

import (
	"cloud.google.com/go/storage"
	"context"
	"github.com/minio/minio-go"
	"google.golang.org/api/iterator"
	"log"
	"os"
)

const(
	minioEndpoint = "minio.my-domain.com"
	minioAccessKeyID = ""
	minioSecretAccessKey = ""
	minioUseSSL = true
	minioTargetBucket = "target-bucket"
	gcsSourceBucket = "source-bucket"
)

func getGCSClient() *storage.Client {
	ctx := context.Background()
	client, err := storage.NewClient(ctx)

	if err != nil {
		log.Fatalln(err)
	}

	return client
}

func getMinioClient() *minio.Client {
	minioClient, err := minio.New(minioEndpoint, minioAccessKeyID, minioSecretAccessKey, minioUseSSL)

	if err != nil {
		log.Fatalln(err)
	}

	return minioClient
}

func getGCSBucketObjectNames(bucket *storage.BucketHandle) []string {
	var names []string
	ctx := context.Background()
	it := bucket.Objects(ctx, &storage.Query{Prefix: ""})

	for {
		attrs, err := it.Next()

		if err == iterator.Done {
			break
		}

		if err != nil {
			log.Fatalln(err)
		}

		names = append(names, attrs.Name)
	}

	return names
}

func copyFile(bucket *storage.BucketHandle, minioClient *minio.Client, name string) {
	log.Printf("Copying object: %s", name)
	ctx := context.Background()
	reader, err := bucket.Object(name).NewReader(ctx)

	if err != nil {
		log.Fatalln(err)
	}

	defer reader.Close()
	_, err = minioClient.PutObject(minioTargetBucket, name, reader, -1, minio.PutObjectOptions{})

	if err != nil {
		log.Fatalln(err)
	}
}

func main() {
	os.Setenv("GOOGLE_APPLICATION_CREDENTIALS", "./gcs_credentials.json")
	gcsClient := getGCSClient()
	gcsBucket := gcsClient.Bucket(gcsSourceBucket)
	gcsObjectNames := getGCSBucketObjectNames(gcsBucket)
	minioClient := getMinioClient()

	for _, name := range gcsObjectNames {
		copyFile(gcsBucket, minioClient, name)
	}
}