Hi joaocandre
I've tested the ADC of my Galileo and I have found out that the A0, A1, A2, A3 are giving values below the 1023 that they should provide on 5V, but the pin A4 and A5 are working properly. I looked at the schematics of the Galileo and the only difference I noticed is that those pins are also connected to another multiplexer for the SDA and SCL. Maybe that could be causing the issue, but let me make some more research about it since I could be wrong. In the meantime I recommend you to use A4 and A5 pins for the Analog digital conversion.
Regards
JPMontero_Intel