是的，C#中的HttpRequestMessage類支持GET請求。可以通過設置HttpRequestMessage對象的Method屬性為HttpMethod.Get來發送GET請求。例如：

using System;
using System.Net.Http;
using System.Threading.Tasks;

class Program
{
    static async Task Main(string[] args)
    {
        using (HttpClient client = new HttpClient())
        {
            HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Get, "https://api.example.com/data");
            HttpResponseMessage response = await client.SendAsync(request);

            if (response.IsSuccessStatusCode)
            {
                string responseBody = await response.Content.ReadAsStringAsync();
                Console.WriteLine(responseBody);
            }
            else
            {
                Console.WriteLine("Request failed with status code: " + response.StatusCode);
            }
        }
    }
}

在上面的示例中，我們創建了一個GET請求，指定了請求的URL為"https://api.example.com/data"。然后使用HttpClient發送該請求，并等待響應。如果響應成功，則將響應內容打印到控制臺上。