  We report an unusual giant linear magnetostrictive effect in the ferrimagnet
Gd$_{2/3}$Ca$_{1/3}$MnO$_3$ ($T_{c} \approx$80 K). Remarkably, the
magnetostriction, negative at high temperature ($T \approx T_{c}$), becomes
positive below 15 K when the magnetization of the Gd sublattice overcomes the
magnetization of the Mn sublattice. A rather simple model where the magnetic
energy competes against the elastic energy gives a good account of the observed
results and confirms that Gd plays a crucial role in this unusual observation.
Unlike previous works in manganites where only striction associated with 3$d$
Mn orbitals is considered, our results show that the lanthanide 4$f$ orbitals
related striction can be very important too and it cannot be disregarded.
