  Magnetoresistance (MR) has attracted tremendous attention for possible
technological applications. Understanding the role of magnetism in manipulating
MR may in turn steer the searching for new applicable MR materials. Here we
show that antiferromagnetic (AFM) GdSi metal displays an anisotropic positive
MR value (PMRV), up to $\sim$ 415%, accompanied by a large negative thermal
volume expansion (NTVE). Around $T_\text{N}$ the PMRV translates to negative,
down to $\sim$ -10.5%. Their theory-breaking magnetic-field dependencies [PMRV:
dominantly linear; negative MR value (NMRV): quadratic] and the unusual NTVE
indicate that PMRV is induced by the formation of magnetic polarons in 5$d$
bands, whereas NMRV is possibly due to abated electron-spin scattering
resulting from magnetic-field-aligned local 4$f$ spins. Our results may open up
a new avenue of searching for giant MR materials by suppressing the AFM
transition temperature, opposite the case in manganites, and provide a
promising approach to novel magnetic and electric devices.
