To reproduce:
-add a RadGroupBox and from the smart tag collapse its header
-add a RadGridView inside the RadGroupBox and dock the grid to fill the parent content. RadGridView does not occupy the whole RadGroupBox content.

Workaround:
public Form1()
{
    InitializeComponent();
 
    this.radGroupBox1.GroupBoxElement.Header.Visibility = Telerik.WinControls.ElementVisibility.Collapsed;
    this.radGridView1.Dock = DockStyle.None;
    this.radGridView1.Location = this.radGroupBox1.Location;
    this.radGridView1.Size = this.radGroupBox1.Size;
    this.radGroupBox1.Resize += radGroupBox1_Resize;  
}
 
private void radGroupBox1_Resize(object sender, EventArgs e)
{
    this.radGridView1.Size = this.radGroupBox1.Size;
}

Second scenario:
- add a RadGroupBox and a RadGridView inside it. Change the RadGridView.Dock property  to Fill.
- increase the GroupBoxElement.Header.Font's size or change the HeaderPosition . The child control does not fill the entire available space. There is empty space at the top of the RadGroupBox.

To reproduce:
- Add a groupbox to a form.
- Set larger than the default font.
- Add a grid to the group box.
- Dock it so it fills the available space.
- The grid is covering the bottom part of the header. 

Workaround:
radGroupBox1.Font = new Font("Segoe UI", 16, FontStyle.Regular);
radGroupBox1.Padding = new Padding(2, radGroupBox1.GroupBoxElement.Header.Size.Height + 2,2, 2);