The GBA SP charger I have is from France. On it it says 230V-50Hz, does that mean that I can't plus it in the USA without a transformer (I have an adapter that is only converting the plug's pin shape).
If the power adapter is specifically labelled as a 230v, it would be unwise to directly plug it in to a 110v plug, such as those in the USA.
You have the option to use a transformer to convert from 110v to 230v, or you could simply purchase another GBA power adapter in the United States.
I double checked on google and found this post from 2005, on tomshardware.com, from a user trying to do the reverse (American GBA in Europe).
The power adapters are different, but the connection to the GBA should be the same. If you can find a cheap power adapter in the states, it should already be regulated for 110v, and will work properly with your GBA SP.