The total number of Bitcoins is capped at 21 million. So how it can handle GDP growth?

The amount of currency should reflect GDP size. Otherwise there will be deflation.

Gold worked well as currency. Its yearly production increased total amount of mined gold by about 3%. Which was on GDP increase level.

if GDP grows such that the smallest subdivision of one bitcoin (called 1 satoshi) becomes too expensive, extra decimals are added. So instead of making new coins, the existing coins will be subdivided in smaller pieces.

I known that it is possible to lower prices denominated in Bitcoins. However this is the deflation problem I am taking about.