You can find the GetGameAPI function in the g_main.c file. This is the only function exported from the gamex86.dll. It is used for exchanging two sets of function pointers between the game DLL and the server. A game_export_t struct is set up by the DLL to point at a dozen functions inside the game DLL. The GetGameAPI function also stores away a pointer to a game_import_t struct, received from the server, in the global gi pointer. The gi pointer is used through out the game DLL to communicate things back to the server.
The export interface consists of a number of functions, which are used more or less infrequently for initializing the game, initializing a new player, exiting game or disconnection a player and changing info about a player like skin, name, etc.
Two functions of the game_export_t are used as entry points for driving the game forward: RunFrame and ClientThink functions. RunFrame is setup to point at the G_RunFrame function while ClientThink points at ClientThink in p_client.c.