GDP growth is always measured in terms of local currency.
It is, therefore, quite possible for a local economy to be experiencing growth whilst its currency is devaluing. This has been the experience of Australia in recent times (circa mid 2000 through mid-2002).
Devaluation of a currency can assist GDP growth primarily through applying a break to imports, and a kick-on for exports. This is sometimes why the Australian media /commentators have referred to the australian economy being super-charged at or about US50c.
Any re-alignment for GDP growth, versus FX variation will be accounted for in terms of the external re-balancing.
Your suggestion of a 5% devaluation in the USD, vs a 3% GDP growth rate does not, however, automatically translate to a 2% contraction, when measured against other currencies.
True enough, various global currencies are often used to measure the relative value of something. But, always, in doing so, it is brought back to measuring the local conditions.
The basic rule for GDP growth, therefore, is change in the nominal value of GDP (ie: the aggregate value of GDP), adjusted for inflation (usually measured in what is called "constant prices" such as 1996 prices).
To get a more meaningful assessment as to the relative value and performance of a particular country's GDP againstd that of another country, it is often useful to look at measures such as: 1) re-valuing GDP in USD terms for global comparative purposes; or 2) measuring GDP in PPP (purchasing price parity) terms (which is used by the Economist Intelligence Unit in its various Country Forecasts); or 3) measuring GDP, for comparative purposes, against a trade basket (such as the Trade Weighted Index which is generally relied upon by Australia in terms of measuring our FX performance against a basket of currencies which broadly reflect our global trading patterns).
