How to change the angle of a gradient layer without changing anything else?

To add to the above, you can not just set the angle. If you want to use other existing settings you need to get them from the layer. The code in that other thread shows one way to get them.

But if you do not need the existing values for any other reason you can get the descriptor for the layer, make a duplicate of it with all the settings and just overwrite the ones you want to change.

This is one way to just change the angle while keeping the other existing settings.

// helper function for working with descriptors
function getProperty( psClass, psKey, index ){// integer:Class, integer:key
    var ref = new ActionReference();
    if( psKey != undefined ) ref.putProperty( charIDToTypeID( "Prpr" ), psKey );
    if(index != undefined ){
        ref.putIndex( psClass, index );
    }else{
        ref.putEnumerated( psClass , charIDToTypeID( "Ordn" ), charIDToTypeID( "Trgt" ) );
    }
    try{
        var desc = executeActionGet(ref);
    }catch(e){ return; }// return on error
    if(desc.count == 0) return;// return undefined if property doesn't exists
    var dataType = desc.getType(psKey);
    switch(dataType){// not all types supported - returns undefined if not supported
        case DescValueType.INTEGERTYPE:
            return desc.getInteger(psKey);
            break;
        case DescValueType.ALIASTYPE:
            return desc.getPath(psKey);
            break;
        case DescValueType.BOOLEANTYPE:
            return desc.getBoolean(psKey);
            break;
        case DescValueType.BOOLEANTYPE:
            return desc.getBoolean(psKey);
            break;
        case DescValueType.UNITDOUBLE:
            return desc.getUnitDoubleValue(psKey);
            break;
        case DescValueType.STRINGTYPE:
            return desc.getString(psKey);
            break;
        case  DescValueType.OBJECTTYPE:
            return desc.getObjectValue(psKey);
            break;
        case  DescValueType.LISTTYPE:
            return desc.getList(psKey);
            break;
        case  DescValueType.ENUMERATEDTYPE:
            return desc.getEnumerationValue(psKey);
            break;
    }
};
function duplicateDescriptor( descriptor ) {
    var newDescriptor = new ActionDescriptor;
    newDescriptor.fromStream( descriptor.toStream() );
    return newDescriptor;
};
function localizeDescriptor( desc ) {
    var stream, pointer, zStringLength, zstring, localized_string, newZStringLength, previousStream, followingStream, newDesc;
    stream = desc.toStream();
    while( true ) {
        pointer = stream.search(/TEXT....\x00\$\x00\$\x00\$/);
        if( pointer === -1 ) {
            break;
        }
        zStringLength = getLongFromStream( stream, pointer + 4 );
        zstring = readUnicode( stream.substr( pointer + 8, ( zStringLength - 1 ) * 2) );
        localized_string = ( localize( zstring ) ) + '\u0000';
        newZStringLength = localized_string.length;
        previousStream = stream.slice( 0, pointer);
        followingStream = stream.slice( pointer + 8 + zStringLength * 2);
        stream = previousStream.concat( 'TEXT', longToString( newZStringLength ), bytesToUnicode( localized_string ), followingStream );
    }
    newDesc = new ActionDescriptor();
    newDesc.fromStream( stream );
    return newDesc;
};
function getShortFromStream( stream, pointer ) {
    var hi, low;
    hi = stream.charCodeAt( pointer ) << 8 ;
    low = stream.charCodeAt( pointer + 1 );
    return hi + low;
 };
function getLongFromStream( stream, pointer ) {
    var hi, low;
    hi = getShortFromStream( stream, pointer) << 16;
    low = getShortFromStream( stream, pointer + 2);
    return hi + low;
};
function readUnicode( unicode ) {
    var string = "";
    for( i = pointer = 0; pointer < unicode.length; i = pointer += 2) {
        string +=String.fromCharCode( getShortFromStream( unicode, pointer ) );
    }
    return string;
};
function longToString( longInteger ) {
    var string;
    string = String.fromCharCode( longInteger >>> 24 );
    string += String.fromCharCode( longInteger << 8 >>> 24 );
    string += String.fromCharCode( longInteger << 16 >>> 24 );
    string += String.fromCharCode( longInteger << 24 >>> 24 );
    return string;
};
function bytesToUnicode( bytes ) {
    var unicode = "", char_code, charIndex;
    for( charIndex  = 0; charIndex < bytes.length; charIndex ++ ) {
        char_code = bytes.charCodeAt( charIndex );
        unicode += String.fromCharCode(char_code >> 8 ) +  String.fromCharCode( char_code & 0xff );
    }
    return unicode;
};
function setGradientAdjustmentAngle( angle ) {
    var adjustmentDesc = getProperty( charIDToTypeID("Lyr "), charIDToTypeID( 'Adjs' ) ).getObjectValue(0);
    var newAdjustmentDesc = duplicateDescriptor( adjustmentDesc );
    newAdjustmentDesc.putUnitDouble( charIDToTypeID('Angl'), charIDToTypeID('#Ang'), angle );
    
    var desc = new ActionDescriptor();
        var ref = new ActionReference();
        ref.putEnumerated( stringIDToTypeID('contentLayer'), charIDToTypeID('Ordn'), charIDToTypeID('Trgt') );
    desc.putReference( charIDToTypeID('null'), ref );
    desc.putObject( charIDToTypeID('T   '), stringIDToTypeID('gradientLayer'), newAdjustmentDesc);
    executeAction( charIDToTypeID('setd'), desc, DialogModes.NO );
};
var newAngle = 45;
setGradientAdjustmentAngle( newAngle );