I have not worked with this notation, so the following is probably not completely correct, but it shows the idea.

Code:

7. ......... ∀x Gx Pr1 6 MP
8. ......... Show ∀x (Gx ∨ Hx)
9. ............ Show Gx ∨ Hx
10. ............ Gx 7 UI
11. ............ Gx ∨ Hx 8 ADD
12. ......... 11 UD
13. ......... ∀x Jx Pr2 12 MP
etc.

You proved ∀x Gx in step 7, and now you need to prove ∀x (Gx ∨ Hx) to use Pr2. Currently, there are no suitable objects to instantiate x with in ∀x Gx. When you start proving ∀x (Gx ∨ Hx), you fix some x (the first step in the proof of a universal statement) and with that particular x you instantiate ∀x Gx.