While these basses are not made by GEWA, they are distributed by them. While I have not seen any of these instruments directly, I have always found that the GEWA name is associated with quality.
They have several models starting from ca. 1500 to ca. 10,000 and more. I've played a 3000 model. It was a good student model, but nothing more. I don't know anything about slap style, but I agree with Jeff that Gewa is usually associated with a good price/quality ratio. Of course a 1500$ bass will not sound like a 10,000$ bass. As always, try before you buy! KN