In the period Jan-Jun 2010 BMW sold 12k of 5er GTs worldwide (2k per month), and 15k since the model entered markets in the last few days of October 2009.
Those numbers are similar to sales figures of eg. 1-series coupe (12k), 1-series convertible (14k), Z4 (14k). Which is not bad for a niche car of that size and price. While eg. X6 sells almost twice as good: 22k units.
BMW plans regarding 5-series GT are still in place: GT sales representing 10% of whole 5-series family sales (sedan + Touring + GT). Right now (Jan-Jun 2010) the percentage is even a bit higher (12.5%), mostly due to sedan and touring Mk change.