Use ampersand && to use AND condition and double pipes || to use OR condition.
<gel:script
xmlns:core="jelly:core"
xmlns:gel="jelly:com.niku.union.gel.GELTagLibrary">
<core:set var="color1" value="blue"/>
<core:set var="color2" value="red"/>
<core:if test="${color1 == 'blue' || color2 == 'notred'}">
<gel:out>OR Condition works !</gel:out>
</core:if>
<core:if test="${color1 == 'blue' && color2 == 'red'}">
<gel:out>AND Condition works, too !</gel:out>
</core:if>
</gel:script>
Output -
OR Condition works !
AND Condition works, too !