The interior texture of the mold will affect the final texture and amount of gloss on your piece. Using an investment mix or mold material with a fine grain will reduce the amount of texture and increase the degree of gloss.
There are also ways to add gloss to a piece that has already been cast. The best method for doing this will depend on the exact nature of the piece.
You could firepolish the piece, which involves returning it to the kiln. Bear in mind that firepolishing happens at slumping temperatures, so it’s not recommended for pieces that will easily deform in this temperature range.
In cases where firepolishing isn’t practical, you may be able to apply a liquid sealer, such as Invisible Shield. These products can add luster to matte surfaces and brighten the appearance of colored glass.