Change of pressure, temperature, carrier gas, carrier flow, fouling of the column, or a chemical enhancing one parameter of the media.

Thank you sir. 

How come, that the pipe inside GC is in circles? Is that to promote the evaporation, and are there other ways, for instance applying heat at various points along  a straight pipe?

Ma`am,

Am not really an expert on GC to answer that question but to my understanding, the column is already inside an oven, distributing an equal amount of heat. The length of the column is related to the elution time of different species. 

If the pipe you are talking is GC caplilary columns, their length could be up to 30 meters long and the only possible way to put them into the GC oven is wind them up. Injected extract is evaporated in inlet liner. The temperature inside oven should be homogenous because a fan is blowing inside. Applying heat or temperature programming will be apply to the whole column and different temperature will affect the partition of analytes between the two phases i.e carrier gas phase and the boned liquid phase. Higher boiling point analyte tends to partion to liquid stationary phase more and hence elute later.