The data format for the gauge is very straightforward: http://code.google.com/apis/visualization/documentation/gallery/gauge.html

Your table should contain two columns: A string label value and a number value. 

Here's some sample controller code:

public String getAverageCostPerCustomer(){
GoogleViz2 gv = new GoogleViz2();

gv.cols = new list<GoogleViz2.col> {
new GoogleViz2.Col('A','Label','string'),
new GoogleViz2.Col('B','Value','number')
};

Map<Id, Decimal> costsByCustomerMap = new Map<Id, Decimal>();

for(Account a : accounts){
Decimal accountCost = 0;
for(Cost_Information__c c : a.Cost_Information__r){
accountCost += c.Amount__c;
}

costsByCustomerMap.put(a.Id, accountCost);
}

Decimal totalCost = 0;
for (Id key : costsByCustomerMap.keySet()) {
Decimal accountCost = costsByCustomerMap.get(key);

totalCost += accountCost;
}

Decimal avgCost = totalCost / accounts.size();

GoogleViz2.row row = new GoogleViz2.row();
row.cells.add ( new GoogleViz2.cell( 'Cost' ) );
row.cells.add ( new GoogleViz2.cell( avgCost ) );
gv.addRow( row );

string results = gv.toJsonString();

system.debug('\n\n\ngv.toJsonString() = ' + results);

return results;
}

 The markup to include the Gauge component is:

<c:Gauge jsondata="{!AverageCostPerCustomer}"

title="Average Cost per Customer"

min="200000" max="300000"

redFrom="275000" redTo="300000"

yellowFrom="250000" yellowTo="275000"

greenFrom="200000" greenTo="250000"/>