The Wikipedia defines potencial GDP as "the total capacity of a nation to produce goods and services". If this definition is correct than the potential GDP can only occur when the unemployment rate is 0, all the land is exploited for the purposes of production (agriculture, factories, roads so on.) and the infrastructure is not outdated.
Speaking primitively and according to the definition provided above, if country A still has some part of it covered by forests (i.e. not factories and roads) and part of the population (say 4 percent) are out of work, then potential output can not be reached i.e. output gap is negative. This hypothetical example describes basically all countries present in the real world.
However, OECD indicates (the link below) that number of countries (Belgium, Denmark, Czech Republic just to name a few) had their real GDPs above the potential. In non of the cases the unemployment rate is O or the land is fully exploited.
Are my assumptions wrong? How should I look to the potencial GDP?