Use a variable from another method in C#

You should read up on scoping... http://msdn.microsoft.com/en-us/library/ms973875.aspx

One way is for your _Qd and _Gd to be at the class level, not defined within the methods themselves, so that you have access to them in the click method.

private decimal _Gd;
private decimal _Qd;
public void readG_TextChanged(object sender, EventArgs e)
{
    string _G = readG.Text;
    _Gd = Convert.ToDecimal(_G);
}

public void readQ_TextChanged(object sender, EventArgs e)
{
    string _Q = readQ.Text;
    _Qd = Convert.ToDecimal(_Q);
}
private void button1_Click(object sender, EventArgs e)
{
    decimal _ULS = (1.35m * _Gd + 1.5m * _Qd);
    Console.WriteLine("{0}",_ULS);
}

This concerns variable scope. The variables _Qd and _Gd only have scope within their methods.

You could make them class members, that is declare them outside your methods, in the main body of the class, as follows:

 private decimal _Gd;
 private decimal _Qd;

.

Then you can set them like this:

 _Gd = Convert.ToDecimal(_G);
 _Qd = Convert.ToDecimal(_Q);

These variables will be visible from within any method in your class.