You need to get the radio button at that index, then get the value of the text of that button. Try this code below.
if(rg1.getCheckedRadioButtonId()!=-1){
int id= rg1.getCheckedRadioButtonId();
View radioButton = rg1.findViewById(id);
int radioId = radioGroup.indexOfChild(radioButton);
RadioButton btn = (RadioButton) rg1.getChildAt(radioId);
String selection = (String) btn.getText();
}