How can I position my dynamically generated GUI boxes at the center of the screen using the screen resolution?

Hey guys, I tried making an Inventory system and I'm having this problem where I create boxes dynamically to make it look like a grid, but seem to be stuck in place no matter what the screen size is.

Here's what I currently have now:

 void DrawInventory()
     {
         Event e = Event.current;
         int i = 0;
 
         for (int y = 0; y < slotsY; y++)
         {
             for (int x = 0; x < slotsX; x++)
             {
                 Rect slotRect = new Rect(x * 60, y * 60, 40, 40);
                 GUI.Box(slotRect, "", skin.GetStyle("Slot"));
                 slots[i] = inventory[i];
 
                 item itemz  = slots[i];
 
                 i++;
             }
     
         }
 
     }

Say I assigned 5 for slotX and 3 for slotY, that would look like this:

Then I tried to center it by changing the Rect's values X and Y statically but obviously that would not be a responsive UI. I tried googling for ways but i couldnt find a solution for centering a GUI boxes generated from a for loop. I'm still new to Unity's GUI feature so if you could please provide a solution for this bug/ point me in the right direction, i would really appreciate it. Thanks everyone.

P.S. Credits for AwfulMedia's YT channel for their free tutorials. That's where i got this code.