The current version of getFXSentimentIndex is as follows:
getFXSentimentIndex(Instrument instrument, Period period, long from, long to)

This version returns null values during weekends. An improvement would be to allow filtering, such as:
getFXSentimentIndex(Instrument instrument, Period period, Filter filter, long from, long to)

The following version would also be useful:
getFXSentimentIndex(Instrument instrument, Period period, Filter filter, int numberOfCandlesBefore, long time, int numberOfCandlesAfter)

Consider calculating the sentiment index as indicator, see:
https://www.dukascopy.com/wiki/#Sentiment_index/Call_Sentiment_Index_as_indicator
instead of using the from-to calculateIndicator method, use one which uses Filter.

Is it possible to get the a currency's sentiment (ie. GBP) instead of a symbol via indicator? If so, can you please demonstrate how this is possible?

Change SentimentIndexBarIndicator.DataSource in the optional inputs.

Would this work?

new Object[] { 
            SentimentIndexBarIndicator.SentimentMode.CHART_PERIOD.ordinal(),
            SentimentIndexBarIndicator.DataSource.Currency.getInstance("GBP"), 
            SentimentIndexBarIndicator.MAX_BARS_DEFAULT 
}

You have to select a value from the SentimentIndexBarIndicator.DataSource enumeration, which you can find in the indicator sources, see:
https://www.dukascopy.com/wiki/#Platform_indicator_source_files

Based on the Netbeans, it looks like the coding would be:

Object[] values2 = indicators.calculateIndicator(instrument, period, new OfferSide[] { OfferSide.ASK }, "SentimentBar",
        new AppliedPrice[] { IIndicators.AppliedPrice.CLOSE },
        new Object[] { 
        SentimentIndexBarIndicator.SentimentMode.CHART_PERIOD.ordinal(), 
        SentimentIndexBarIndicator.DataSource.PRIMARY_CURRENCY.ordinal(), 
        SentimentIndexBarIndicator.MAX_BARS_DEFAULT 
}, from, to);

To get the GBP, I would need to use an instrument such as the GBP/USD.

Is this correct?

Test the code and report if any issues.