Top content isn't a metric, it's just a list of the pages on your site with the highest number of page views.
The metric you're looking for is 'entranceBounceRate' and the dimension is 'pagePath'. You want to get the bounce rate for the top X most visited pages on your site, so you'll want to limit your results and sort the results by '-pageviews' (pageviews descending).
If you want to get the bounce rate for the top 10 most viewed pages on your site, your query should look like this:
$ga = new gapi('[email protected]','password');
$ga->requestReportData(145141242,array('pagePath'),array('entranceBounceRate','pageviews'),array('-visits'),null,null,null,10);
The Google Analytics Export API has a data feed query explorer that should help you out considerably when using GAPI:
http://code.google.com/apis/analytics/docs/gdata/gdataExplorer.html
Also, here's a list of all available dimensions and metrics you can pull from the API:
http://code.google.com/apis/analytics/docs/gdata/gdataReferenceDimensionsMetrics.html
Definitely read over the GAPI documentation:
http://code.google.com/p/gapi-google-analytics-php-interface/wiki/GAPIDocumentation