@mixin verticalGradient($fromColor, $toColor) {
    background-color: $toColor;
    background-image: -moz-linear-gradient(top, $fromColor, $toColor);
    ...
    background-image: linear-gradient(to bottom, $fromColor, $toColor);
}
.btn-b {
    @include verticalGradient(blue, red);
}