Here is a tip: Never get excited to steal/mug/win/aquire materia from a battle at the "Winnings" screen. Battles never give you materia, unless a piece of materia falls out, which also happens only after a special battle which only is fought once. i.e, The Gi Nattak, which you fight once and it dies, gives you the first piece of "Gravity" Materia. This is either the first or second piece. The other one might be what you get from the weird battle in Hojo's lab (When Red XIII joins your team. Or is that Enemy Skill? Most likely.