Hi,
“Index” is actually a function that allow you to obtain the difference divided by the sum:

https://docs.sentinel-hub.com/api/latest/evalscript/functions/

To obtain what you want, you just need to substitute that line with the formula you want (and also fix input and output. Like this:

indices_evalscript = """
                        //VERSION=3

                        function setup() {
                            return {
                                input: [{
                                    bands:[“B03”,“B08”,“dataMask”]}],
                                output:[{
                                    id: "GCVI",
                                    bands: 1
                                },
                                    {
                                id: “dataMask”,
                                bands: 1
                                }]
                            }
                        }

                        function evaluatePixel(sample) {
                            return {
                                GCVI: [(samples.B08/samples.B03)-1],
                                dataMask: [samples.dataMask]
                            };
                        }
                    """

I guess that this should work

indices_evalscript = """
                        //VERSION=3

                        function setup() {
                            return {
                                input: [{
                                    bands:[“B03”,“B08”,“dataMask”]}],
                                output:[{
                                    id: "GCVI",
                                    bands: 1
                                },
                                    {
                                id: “dataMask”,
                                bands: 1
                                }]
                            }
                        }

                        function evaluatePixel(sample) {
                            return {
                                GCVI: [(samples.B08/samples.B03)-1],
                                dataMask: [samples.dataMask]
                            };
                        }
                    """

Thanks! I tried it but getting the following error:
sentinelhub.exceptions.DownloadFailedException: Failed to download from: https://services.sentinel-hub.com/api/v1/statistics with HTTPError: 400 Client Error: Bad Request for url: https://services.sentinel-hub.com/api/v1/statistics Server response: "{"status": 400, "reason": "Bad Request", "message": "Failed to evaluate script!\nevalscript.js:7: SyntaxError: Invalid or unexpected token\n

Any idea about what might be going wrong?

There is a missing “s” in “function evaluatePixel(sample)”

try to substitute with
“function evaluatePixel(samples)”

That worked. Also, I had to change the quotes around dataMask to match those around GCVI. Thank you!