GFs take time to summon, rendering the caller inactive until the summon completes. A GF can be knocked out in battle, making it useless until again revived. Early in the game, GFs are so much more powerful than your characters that you'll let them do the dirty work. Later, however, the balance of power shifts back to your characters, keeping the battles from becoming endless summonfests. Enemies remain on your level throughout the game, evolving to learn new attacks and new spells.
Victory is more dependent on careful junctioning than sheer numerical superiority.
Spells are Drawn from foes while in battle, or from Draw Points scattered throughout the environments. Moreover, every time you use a spell that's Junctioned to an attribute, that attribute is weakened. Final Fantasy VIII achieves an excellent balance between attacks, magic, and GFs, requiring both thought and a sound strategy at all times. One of the larger criticisms aimed at Final Fantasy VII was that players could sail through the game by purchasing new weapons and ignoring magic. Many people were also disappointed with the ease of Final Fantasy VII's endgame.