gist tag plugin now works with the latest changes GitHub gists and does not fail if you do not specify a filename.

.themes/classic/sass/partials/_syntax.scss

@@ -1,6 +1,8 @@
 .highlight, html .gist .gist-file .gist-syntax .gist-highlight {
   table td.code { width: 100%; }
-  .line-numbers {
+  border: 1px solid $pre-border !important;
+}
+.highlight .line-numbers, html .gist .gist-file .gist-syntax .highlight .line_numbers {
   text-align: right;
   font-size: 13px;
   line-height: 1.45em;
@@ -18,27 +20,33 @@
   span { color: $base01 !important; }
   padding: .8em !important;
   @include border-radius(0);
-  }
-  border: 1px solid $pre-border !important;
 }
+
 figure.code, .gist-file, pre {
   @include box-shadow(rgba(#000, .06) 0 0 10px);
   .highlight pre { @include box-shadow(none); }
 }
 
+.gist .highlight, figure.code .highlight {
+  @include selection(adjust-color($base03, $lightness: 23%, $saturation: -65%), $text-shadow: $base03 0 1px);
+}
 html .gist .gist-file {
   margin-bottom: 1.8em;
   position: relative;
   border: none;
   padding-top: image-height("code_bg.png") !important;
+  .highlight {
+    margin-bottom: 0;
+  }
   .gist-syntax {
     border-bottom: 0 !important;
     background: none !important;
-    .gist-highlight{
+    .gist-highlight {
       background: $base03 !important;
-      pre {
-        @extend .pre-code;
     }
+    .highlight pre {
+      @extend .pre-code;
+      padding: 0;
     }
   }
   .gist-meta {
@@ -107,12 +115,11 @@ p, li {
 }
 
 .pre-code {
-  @include selection(adjust-color($base03, $lightness: 23%, $saturation: -65%), $text-shadow: $base03 0 1px);
   font-family: $mono !important;
   overflow: scroll;
   overflow-y: hidden;
   display: block;
-  padding: .8em !important;
+  padding: .8em;
   overflow-x: auto;
   line-height: 1.45em;
   background: $base03 $noise-bg !important;
@@ -187,7 +194,7 @@ p, li {
 }
 
 .highlight, .gist-highlight {
-  pre { background: none; @include border-radius(none); border: none; padding: 0; margin-bottom: 0; }
+  pre { background: none; @include border-radius(0px); border: none; padding: 0; margin-bottom: 0; }
   margin-bottom: 1.8em;
   background: $base03;
   overflow-y: hidden;
@@ -206,7 +213,9 @@ pre, .highlight, .gist-highlight {
   &::-webkit-scrollbar-thumb:horizontal { background: $solar-scroll-thumb;  -webkit-border-radius: 4px; border-radius: 4px }
 }
 
-.highlight code { @extend .pre-code; background: #000;}
+.highlight code { 
+  @extend .pre-code; background: #000;
+}
 figure.code {
   background: none;
   padding: 0;
@@ -250,4 +259,3 @@ figure.code {
   text-shadow: #cbcccc 0 1px 0;
   padding-left: 3em;
 }
-

plugins/gist_tag.rb

@@ -40,7 +40,9 @@ module Jekyll
     end
 
     def script_url_for(gist_id, filename)
-      "https://gist.github.com/#{gist_id}.js?file=#{filename}"
+      url = "https://gist.github.com/#{gist_id}.js"
+      url = "#{url}?file=#{filename}" unless filename.nil? or filename.empty?
+      url
     end
 
     def get_gist_url_for(gist, file)