Sadly, you really can’t transition gradients for now.
So, the only workable workaround is using an extra element with needed gradient and transition it’s opacity:
a.button {
position: relative;
z-index: 9;
display: inline-block;
padding: 0 10px;
background: -webkit-gradient(linear, 0 0, 0 100%, from(green), to(#a5c956));
background: -webkit-linear-gradient(top, green, #a5c956);
background: -moz-linear-gradient(top, green, #a5c956);
background: -o-linear-gradient(top, green, #a5c956);
background: linear-gradient(top, green, #a5c956);
}
.button-helper {
position: absolute;
z-index: -1;
top: 0;
left: 0;
width: 100%;
height: 100%;
opacity: 0;
background: -webkit-gradient(linear, 0 0, 0 100%, from(lime), to(#89af37));
background: -webkit-linear-gradient(top, lime, #89af37);
background: -moz-linear-gradient(top, lime, #89af37);
background: -o-linear-gradient(top, lime, #89af37);
background: linear-gradient(top, lime, #89af37);
-webkit-transition: opacity 1s linear;
-moz-transition: opacity 1s linear;
-o-transition: opacity 1s linear;
transition: opacity 1s linear;
}
a.button:hover .button-helper {
opacity: 1;
}
<a href="#" class="button"><span class="button-helper"></span>button</a>