Progressive Group Task (PGT) is the first of the GTO task in which all GTO Rules apply, this task consists of 4 obstacles in a straight line. It is known as a group task because all of you will be working as a group to acheive a common goal. The task is called progressive because the first obstacle is very very very simple, the second obstacle is very very simple, the third obstacle is very simple and the fourth one is simple, this means that the level of difficulty increases with obstacles and hence the name Progressive.
In Service Selection Boards there are no natural obstacles like Ditches, Nalas and rivers so what is done is that a set of parallel lines are drawn and the area between them is placed out of bounds for candidates. So these becomes obstacles to you, to help you cross the obstacles certain structures are placed which the candidates can touch, you will be provided with helping material in form of planks, Ballies and Ropes. You will be also carrying a load with you which is a sensitive black box whom you have to carry carefully.
GTO outdoor task rules are applicable to other outdoor tasks also but GTO will brief you about the Rules only once, so if you have any doubt regarding any rule then you must clear before the start of PGT.