Respond : it's not the same.
if you try to insert 3DS stylus inside the WiiU GamePad, it will probably cancel the waranty.
Then please try to purchase from Nintendo, a Stylus replacement.
WiiU Stylus
https://store.nintendo.com/ng3/us/po/browse/productDetailColorSizePicker.jsp?productId=prod10883
Nintendo 3DS XL Stylus
https://store.nintendo.com/ng3/us/po/browse/productDetailColorSizePicker.jsp?productId=prod10826
Nintendo 2DS Stylus
https://store.nintendo.com/ng3/us/po/browse/productDetailColorSizePicker.jsp?productId=prod150184
New Nintendo 3DS XL Stylus
https://store.nintendo.com/ng3/us/po/browse/productDetailColorSizePicker.jsp?productId=prod410032
Nintendo 3DS Stylus
https://store.nintendo.com/ng3/us/po/browse/productDetailColorSizePicker.jsp?productId=prod10236