They mean gigawatts of power, at some specific time. Germany has about 24.8 GW [1] of installed solar capacity, which recently peaked about 22 GW of actual power. The time-average is about 2.1 GW as of last year [1] (as 18 TWh/year), because solar panels in Germany only yield about 10-12% of their capacity averaged over the day and year. [a] So this is about the same as a (one) large nuclear reactor in electricity generated, but not peak power.

Incidentally it's cost about €100 billion [2] to build this, at taxpayer expense. (Though this a future-spending figure: the subsidies aren't paid at the time a power plant is built, but deferred over 20 years -- the present value is somewhat lower). The specific subsidy rates are listed here [3] (they're set by the year the plant is installed).

[1] http://en.wikipedia.org/wiki/Solar_power_in_Germany

[2] http://www.spiegel.de/international/germany/inflated-incenti...

[3] http://en.wikipedia.org/wiki/Financial_incentives_for_photov...

[a] Note that the installed capacity increased a lot over 2011, so you can't do simple comparisons like dividing 2.1/24.8 -- a lot of the capacity wasn't installed at the beginning of the year, and was at the end. But see the example capacity factors in the power plant table in [1]