Error: NullPointerException to null object reference (Android)

The only thing you should do is these two steps:

1.- Validate that the parameter you pass is not null since when going to another activity I assume that you execute the method finish (), then every time you return to this activity it tries to consume this variable and as this null, it will generate error .

You can validate by placing:

protected void onCreate(Bundle savedInstanceState) {

   Bundle bundle = this.getIntent().getExtras();
   if(bundle !=null){
      idGestor = bundle.getString("idG");  
   } 
}

2.- You must send from the activity2 in the onback event the idG parameter

@Override
 public void onBackPressed() {
   Intent intent = new Intent(Activity2.this, Activity.class);
   intent.putExtra("idG", idG);
   startActivity(intent)
   finish();
 }

3.- Something I'm not sure about is that in your adapter method you call the parameter: idU

public void onClick(View v) {
Intent inicioIntent = new Intent(context, Activity2.class);
inicioIntent.putExtra("idU", idUser);
context.startActivity(inicioIntent);}

But in your activity2 you want to retrieve the parameter with the name idG

protected void onCreate(Bundle savedInstanceState) {
     idGestor = intent.getExtras().getString("idG");
}

Should not that be the case if it were the case in reality?

protected void onCreate(Bundle savedInstanceState) {
idGestor = intent.getExtras().getString("idU");}

In MainActivity2 you must do the following, add super.onCreate(savedInstanceState); and also validate the receipt of bundle :

protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);

 Bundle parametros = this.getIntent().getExtras();
 if(parametros !=null){
    idGestor = parametros.getString("idG");  
 } 

}