Nazis are an Easter egg in Grand Theft Auto Online.
The real life Nazi Party was a fascist third position group which ruled over Germany between 1933 and 1945. They were ruled throughout this period by Adolf Hitler.
In terms of the GTA series, the Nazis are featured as an easter egg in GTA V's multiplayer mode, Grand Theft Auto Online. They appear in the form of references. The most prominent reference to them is the hair style which was added in the "I Am Not A Hipster" update to GTA Online, the "Hipster Youth" hairstyle. Firstly, "Hipster Youth" is based on the "Hitler Youth", and secondly, the actual hairstyle itself appears to be based on a stereotypical German haircut from the time period. Another smaller reference is the breast-jackets which were added in another update, which are based on Nazi uniforms from WWII.
Using the crew feature in the game, players are able to make a Nazi logo to be used in the online mode, but Rockstar stresses when publishing an emblem that if it features racism or something similar, the player will likely be permanently banned from GTA Online.