How do I split text file by lines and group in grid view

If you would like to display data in DataGridView control, then STOP thinking about lines (strings) and START thinking about objects!

Seems, that every line between [ISTK-N] tags is a record (object) with custom fields (properties), like: MAT, L, B, A, REF, BEM, BEM2, EDGE1, EDGE2, EDGE3 and EDGE4. So, you need to create custom class, which will represent that object (record):

public class MyData
{
	public MyData()
	{
	 	//empty constructor;
	}
	
	public MyData(string sFileName)
	{
	 	LoadData(sFileName);
	}
	
	public string ISTK { get; set;}
	public string MAT { get; set;}
	public int L { get; set;}
	public int B { get; set;}
	public int A { get; set;}
	public string REF { get; set;}
	public string BEM { get; set;}
	public string BEM2 { get; set;}
	public string EDGE1 { get; set;}
	public string EDGE2 { get; set;}
	public string EDGE3 { get; set;}
	public string EDGE4 { get; set;}
	
	public List<MyData> LoadData(string sFileName)
	{
		List<MyData> internallist = new List<MyData>();
		MyData md = null;
		List<string> lines = File.ReadAllLines(sFileName).Where(x=>x.Trim().Length>0).ToList();
		string pattern = @"\[ISTK\-\d{1,}\]";
		Regex r = new Regex(pattern);
		foreach(string line in lines)
		{
			if (r.IsMatch(line))
			{
				md = new MyData(){ISTK= line.Replace("[", "").Replace("]", "")};
				if(internallist.Where(x=>x.ISTK.Equals(md.ISTK)).SingleOrDefault()==null) internallist.Add(md);
			}
			else
			{
				string[] props = line.Split('=');
				switch(props[0])
				{
					case "MAT":
						md.MAT = props[1];
						break;
					case "L":
						md.L = Convert.ToInt32(props[1]);
						break;
					case "B":
						md.B = Convert.ToInt32(props[1]);
						break;
					case "A":
						md.A = Convert.ToInt32(props[1]);
						break;
					case "REF":
						md.REF = props[1];
						break;
					case "BEM":
						md.BEM = props[1];
						break;
					case "BEM2":
						md.BEM2 = props[1];
						break;
					case "EDGE1":
						md.EDGE1 = props[1];
						break;
					case "EDGE2":
						md.EDGE2 = props[1];
						break;
					case "EDGE3":
						md.EDGE3 = props[1];
						break;
					case "EDGE4":
						md.EDGE4 = props[1];
						break;
				}
			}
			
			
		}
		
		return internallist;
	}

}

Then, you'll be able to use it like this:

List<MyData> mdlist = new MyData().LoadData(@"FullFileName.stk");
DataGridView1.DataSource = mdlist;

That's all!

Note: There's few other ways to achieve that, but it should be enough for start.