Brick Piece, Gold Leaf, Silver Leaf, and Slowpoketail Have NO effect(Just sell them).
The first three can only be obtained by trading cetain species of Pokemon from RGBY to GSC
While Gold Leaf and Silver Leaf had a very high chance of being attached to a RGBY Pokemon... Others were not so easy.
Hard-to-get Items in GSC:
Brick Piece: Trade over RGBY Machop
Light Ball: Trade over THE Yellow Version Pikachu
Twistedspoon: Trade over RGBY Kadabra THAT WAS CAUGHT AS A KADABRA
TM09 Psych Up: Trade over RGB Abra (Not Yellow)
Brightpowder: Trade over Articuno, Zapdos, Moltres, and/or Mewtwo from RGBY
Metal Power: Trade over RGBY Ditto
Polkadot Bow: Trade over RGBY Jigglypuff
Lucky Punch: Trade over RGBY Chansey
Other items that could be obtained included Berry, Bitter Berry, Gold Berry, Mystery Berry, Leftovers, TM 33(Ice Punch), TM 43(Attract), and Protein.