Esta es la mejor herramienta para esto:
http://www.colorzilla.com/gradient-editor/
background: #717172; /* Old browsers */
background: -moz-linear-gradient(top, #717172 0%, #3a3a3a 50%, #0d0d0f 51%, #0f0f10 100%); /* FF3.6+ */
background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,#717172), color-stop(50%,#3a3a3a), color-stop(51%,#0d0d0f), color-stop(100%,#0f0f10)); /* Chrome,Safari4+ */
background: -webkit-linear-gradient(top, #717172 0%,#3a3a3a 50%,#0d0d0f 51%,#0f0f10 100%); /* Chrome10+,Safari5.1+ */
background: -o-linear-gradient(top, #717172 0%,#3a3a3a 50%,#0d0d0f 51%,#0f0f10 100%); /* Opera11.10+ */
background: -ms-linear-gradient(top, #717172 0%,#3a3a3a 50%,#0d0d0f 51%,#0f0f10 100%); /* IE10+ */
filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#717172', endColorstr='#0f0f10',GradientType=0); /* IE6-9 */
background: linear-gradient(top bottom, #717172 0%,#3a3a3a 50%,#0d0d0f 51%,#0f0f10 100%); /* W3C (Firefox 16+) */