G4P version 3 introduced a new class called StyledString which supports various text formatting including super and subscript.

The short sketch below displays the atomic symbol, number(super) and weight(sub) for gold. You should see something like
Au ⁷⁹ _196.97
  EDIT sorry the super/sub only showed when I was editing this post and disappeared when I published it.

Unfortunately the super and subscripts can not be displayed in the same horizontal position I will have to see if I do something that for a future release.

Notice that I am using a GLabel to display the text, I would only use a GTextField for input in which case it has methods for styling any selected/highlighted text (see G4P_TextAreaControl example that comes with the library).

1. import g4p_controls.*;
2. StyledString gold;
3. GLabel l0;
4. 
   
5. void setup() {
6.   size(300, 300);
7.   gold = new StyledString("Au79196.97");
8.   gold.addAttribute(G4P.SUPERSCRIPT, G4P.SUPERSCRIPT_SUPER, 2, 4);
9.   gold.addAttribute(G4P.SUPERSCRIPT, G4P.SUPERSCRIPT_SUB, 4, 10);
10.   l0 = new GLabel(this, 20, 50, 100, 22);
11.   l0.setOpaque(true);
12.   l0.setStyledText(gold);
13. }
14. 
    
15. void draw() {
16.   background(255);
17. }