AG1 for Extra countermeasures
AG2 for Tail Hook
AG3 to Eject wing pylons

Compared to the GF-5A:
-More Missiles
-Moved fuel to the middle to prevent imbalance after they run out

Lore:
The GF-5B variant was originally supposed to just re-add the outer wing pylons from the GF-5(which were scrapped due to issues and time constraint) to the GF-5A. However, the GF-5A had a major imbalance issue that mainly happened when trying to change pitch with fully swept wings. This is due to the fuel tanks being placed at the nose of the plane for balancing, which caused issues when the fuel ran out. It was also an issue in the G-2, though the effects are negligable.

Naturally, the GF-5B variant was required to solve this problem before it can go into production. To solve it, the fuel tanks were moved to the middle, stored mainly in the inner wings. They also added extra dead weight in the nose, which unfortunately caused the GF-5B to be around 270KGs heavier than the GF-5A(Excluding the missiles). But it's a fair price to pay for consistent balance.

The GF-5B became the main variant that's mainly used. Even with the extra weight, it was able to consistently beat the G-2 in both tests and real combat. To safe costs, the existing GF-5A planes didn't get modified further. Instead, the pilots are told to not use forward sweep on low fuel. It having less weight than the GF-5B still made it useful in niche situations.