Belgium Output Gap, % of potential GDP. Units: Percent of potential GDP. Multiplier: . Estimates begin after 2015 . This data is sourced from www.opendataforafrica.org where it is offered under an open data licence (www.opendataforafrica.org/legal/termsofuse). Output gaps for advanced economies are calculated as actual GDP less potential GDP as a percent of potential GDP. Estimates of output gaps are subject to a significant margin of uncertainty. For a discussion of approaches to calculating potential output, see Paula R. De Masi, IMF Estimates of Potential Output: Theory and Practice, in Staff Studies for the World Economic Outlook (Washington: IMF, December 1997), pp. 40-46. See notes for: Gross domestic product, constant prices (National currency)..