Crevette vs. gambas
In French, both crevette and gambas translate to shrimp or prawn, but they are not always interchangeable. The distinction primarily depends on the size of the crustacean and the culinary context in which the word is used.
Crevette
The word crevette is the standard, general term used to describe any shrimp or prawn, regardless of its size or species. It can be used in everyday language to talk about the living marine animal in the ocean, as well as in culinary contexts to describe the seafood on a plate.
J'ai acheté un kilo de crevettes roses au marché.
(I bought a kilo of pink shrimp at the market.)
Les enfants s'amusent à pêcher la crevette dans les flaques d'eau.
(The children are having fun catching shrimp in the puddles.)
Ce restaurant asiatique sert d'excellents beignets de crevettes.
(This Asian restaurant serves excellent prawn fritters.)
Une petite crevette grise s'est échappée du filet de pêche.
(A small grey shrimp escaped from the fishing net.)
Il faut décortiquer chaque crevette avant de l'ajouter à la salade.
(You must peel each prawn before adding it to the salad.)
Gambas
The word gambas is a term borrowed from Spanish that specifically refers to large prawns or king prawns. It is almost exclusively used as a culinary term to describe a premium dish, usually featuring large-sized prawns that are cooked and served whole.
Le chef nous a préparé des gambas flambées au pastis.
(The chef prepared king prawns flambéed with pastis for us.)
Nous avons commandé une assiette de gambas grillées à la plancha pour partager.
(We ordered a plate of grilled large prawns on the griddle to share.)
Ces gambas sauvages de Madagascar ont un goût exceptionnellement riche.
(These wild king prawns from Madagascar have an exceptionally rich taste.)
Pour le repas de fête de ce soir, elle a cuisiné un risotto aux gambas.
(For tonight's festive meal, she cooked a king prawn risotto.)
Il est souvent beaucoup plus facile de manger des gambas entières avec les doigts.
(It is often much easier to eat whole large prawns with your fingers.)
Summary
To summarize, you should use crevette as the universal term for any shrimp or prawn in both nature and everyday cooking. You should reserve the word gambas for restaurant menus and culinary contexts when specifically referring to large, premium king prawns.