Insertion fails when \"\'\" is used. example string is: He\'s is a boy. I\'ve attempted to skip the \"\'\" using an escape symbol , but I believe this is not the right way.<
On the MSDN article for String.Replace it says:
Returns a new string in which all occurrences of a specified Unicode character or String in the current string are replaced with another specified Unicode character or String.
On the very first line you are not assigning the value of textBox3.Text to the result of that method call, meaning that absolutely nothing happens.
Furthermore, to escape a quote in SQL Server, you simply use two single-quotes (Note: NOT the same thing as a double-quote).
This should give you the expected outcome:
textBox3.Text = textBox3.Text.Replace("'", "''");
Additionally, you may wish to look into String.Format for your string concatenation needs.
String escapedInput = textBox3.Text.Replace("'", "''");
String sql = String.Format("insert into gtable (1text,1memo) values ('{0}',null)", escapedInput);