Simple .gsheet to TextAsset ScriptedImporter for Unity, with code from https://gist.github.com/jasonleehodges/359eb2af0d1caec0153bb7450c165ffd

GSheetImporter.cs

[ScriptedImporter(1, "gsheet")]
public class GSheetImporter : ScriptedImporter
{
    #region Types

    private class WebClientEx : WebClient
    {
        #region Fields

        private readonly CookieContainer _container;

        #endregion

        #region Constructors

        public WebClientEx(CookieContainer container)
        {
            _container = container;
        }

        #endregion

        #region Methods

        protected override WebRequest GetWebRequest(Uri address)
        {
            var r = base.GetWebRequest(address);
            if (r is HttpWebRequest request)
            {
                request.CookieContainer = _container;
            }

            return r;
        }

        protected override WebResponse GetWebResponse(WebRequest request, IAsyncResult result)
        {
            var response = base.GetWebResponse(request, result);
            ReadCookies(response);
            return response;
        }

        protected override WebResponse GetWebResponse(WebRequest request)
        {
            var response = base.GetWebResponse(request);
            ReadCookies(response);
            return response;
        }

        private void ReadCookies(WebResponse r)
        {
            if (r is HttpWebResponse response)
            {
                var cookies = response.Cookies;
                _container.Add(cookies);
            }
        }

        #endregion
    }

    [Serializable]
    private class AdditionalSheet
    {
        #region Properties

        [field: SerializeField]
        public string Name { get; private set; }

        [field: SerializeField]
        public int Gid { get; private set; }

        #endregion

        #region Constructors

        public AdditionalSheet(string name, int gid)
        {
            Name = name;
            Gid = gid;
        }

        #endregion
    }

    #endregion

    #region Properties

    [field: SerializeField]
    private AdditionalSheet[] Sheets { get; set; }

    #endregion

    #region Methods

    [OnOpenAsset]
    private static bool OnOpenAsset(int instanceID)
    {
        var asset = EditorUtility.InstanceIDToObject(instanceID);
        var assetPath = AssetDatabase.GetAssetPath(asset);
        if (assetPath.EndsWith(".gsheet"))
        {
            var docId = GetDocId(assetPath);
            var assetImporter = (GSheetImporter)GetAtPath(assetPath);
            var sheet = AssetDatabase.IsMainAsset(asset)
                ? new AdditionalSheet("", 0)
                : assetImporter.Sheets.First(a => a.Name == asset.name);
            var url = $"https://docs.google.com/spreadsheets/d/{docId}/edit#gid={sheet.Gid}";
            Application.OpenURL(url);
            return true;
        }

        return false;
    }

    private static string GetDocId(string assetPath)
    {
        var contents = File.ReadAllText(assetPath);
        var jObject = JObject.Parse(contents);
        var docId = jObject["doc_id"]!.Value<string>();
        return docId;
    }

    /// <inheritdoc />
    public override void OnImportAsset(AssetImportContext ctx)
    {
        var docId = GetDocId(ctx.assetPath);
        var textAsset = GetTextAsset(ctx, docId, new AdditionalSheet(Path.GetFileNameWithoutExtension(ctx.assetPath), 0));
        ctx.AddObjectToAsset("MainObject", textAsset);
        ctx.SetMainObject(textAsset);
        foreach (var sheet in Sheets.Where(s => !string.IsNullOrWhiteSpace(s.Name)))
        {
            textAsset = GetTextAsset(ctx, docId, sheet);
            ctx.AddObjectToAsset(textAsset.name, textAsset);
        }
    }

    private TextAsset GetTextAsset(AssetImportContext ctx, string docId, AdditionalSheet sheet)
    {
        string csvContents;
        try
        {
            var url = $"https://docs.google.com/spreadsheets/d/{docId}/export?format=csv&gid={sheet.Gid}";

            using var wc = new WebClientEx(new CookieContainer());
            wc.Headers.Add("User-Agent", "Mozilla/5.0 (Windows NT 6.2; WOW64; rv:22.0) Gecko/20100101 Firefox/22.0");
            wc.Headers.Add("DNT", "1");
            wc.Headers.Add("Accept", "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8");
            wc.Headers.Add("Accept-Encoding", "deflate");
            wc.Headers.Add("Accept-Language", "en-US,en;q=0.5");

            csvContents = wc.DownloadString(url);
        }
        catch (Exception e)
        {
            ctx.LogImportError(e.ToString());
            csvContents = e.ToString();
        }


        var textAsset = new TextAsset(csvContents)
        {
            name = sheet.Name
        };
        return textAsset;
    }

    #endregion
}

GSheetImporterEditor.cs

[CanEditMultipleObjects]
[CustomEditor(typeof(GSheetImporter))]
public class GSheetImporterEditor : ScriptedImporterEditor
{
    #region Methods

    /// <inheritdoc />
    public override void OnInspectorGUI()
    {
        DrawDefaultInspector();
        if (GUILayout.Button("Reload"))
        {
            SaveChanges();
        }

        ApplyRevertGUI();
    }

    #endregion
}