gedit currently does not support syntax highlighting for Scala (http://www.scala-lang.org/).

This could be a starting point:

<?xml version="1.0" encoding="UTF-8"?>
<language id="scala" _name="Scala" version="2.0" _section="Sources">
  <metadata>
    <property name="mimetypes">text/x-scala</property>
    <property name="globs">*.scala</property>
    <property name="line-comment-start">//</property>
  </metadata>

  <styles>
    <style id="comment" _name="Comment" map-to="def:comment"/>
    <style id="string" _name="String" map-to="def:string"/>
    <style id="keyword" _name="Keyword" map-to="def:keyword"/>
    <style id="decimal" _name="Decimal" map-to="def:decimal"/>
    <style id="specials" _name="Specials" map-to="def:specials"/>
    <style id="data-type" _name="Data Type" map-to="def:data-type"/>
    <style id="others" _name="Others"/>
  </styles>

  <definitions>
    <context id="block-string" style-ref="string">
      <start>"""</start>
      <end>"""</end>
    </context>
    <context id="string" style-ref="string" end-at-line-end="true">
      <start>"</start>
      <end>"</end>
    </context>
    <context id="string-2" style-ref="string" end-at-line-end="true">
      <start>'</start>
      <end>'</end>
    </context>
    <context id="line-comment" style-ref="comment" end-at-line-end="true">
      <start>//</start>
    </context>
    <context id="block-comment" style-ref="comment">
      <start>\/\*</start>
      <end>\*\/</end>
    </context>
    <context id="declarations" style-ref="keyword">
      <keyword>class</keyword>
      <keyword>def</keyword>
      <keyword>extends</keyword>
      <keyword>forSome</keyword>
      <keyword>implicit</keyword>
      <keyword>import</keyword>
      <keyword>mixin</keyword>
      <keyword>object</keyword>
      <keyword>package</keyword>
      <keyword>trait</keyword>
      <keyword>type</keyword>
      <keyword>val</keyword>
      <keyword>var</keyword>
      <keyword>with</keyword>
    </context>
    <context id="primitives" style-ref="data-type">
      <keyword>Boolean</keyword>
      <keyword>Byte</keyword>
      <keyword>Char</keyword>
      <keyword>Double</keyword>
      <keyword>Float</keyword>
      <keyword>Int</keyword>
      <keyword>Long</keyword>
      <keyword>Short</keyword>
    </context>
    <context id="flow" style-ref="keyword">
      <keyword>case</keyword>
      <keyword>catch</keyword>
      <keyword>do</keyword>
      <keyword>else</keyword>
      <keyword>finally</keyword>
      <keyword>for</keyword>
      <keyword>if</keyword>
      <keyword>match</keyword>
      <keyword>requires</keyword>
      <keyword>return</keyword>
      <keyword>throw</keyword>
      <keyword>try</keyword>
      <keyword>while</keyword>
      <keyword>yield</keyword>
    </context>
    <context id="modifiers" style-ref="keyword">
      <keyword>abstract</keyword>
      <keyword>final</keyword>
      <keyword>implicit</keyword>
      <keyword>lazy</keyword>
      <keyword>override</keyword>
      <keyword>private</keyword>
      <keyword>protected</keyword>
      <keyword>sealed</keyword>
    </context>
    <context id="memory" style-ref="others">
      <keyword>false</keyword>
      <keyword>new</keyword>
      <keyword>null</keyword>
      <keyword>super</keyword>
      <keyword>this</keyword>
      <keyword>true</keyword>
    </context>
    <context id="numeric" style-ref="decimal">
      <match>(\b([0-9]+|0[xX][0-9a-fA-F]+)[Ll]?\b|\b(([0-9]+[Ee][-]?[0-9]+|([0-9]*\.[0-9]+|[0-9]+\.)([Ee][-]?[0-9]+)?)[fFdD]?|[0-9]+[FfDd]))</match>
    </context>

    <context id="scala">
      <include>
        <context ref="block-string"/>
        <context ref="string"/>
        <context ref="string-2"/>
        <context ref="line-comment"/>
        <context ref="block-comment"/>
        <context ref="declarations"/>
        <context ref="primitives"/>
        <context ref="flow"/>
        <context ref="modifiers"/>
        <context ref="memory"/>
        <context ref="numeric"/>
      </include>
    </context>
  </definitions>
</language>