CW also mentioned that one can try his GPP ASAP workout for the GPP day/part of some of his programme.
CT's Running Man would also be a good one.
Yes of course. Exercises that keep your heartrate high and bring up your overall fitness (power, endurance etc.) are considered GPP. For example:
Track running (400m, 200m etc.)
Or a mixture of the above.
It depends on your current fitness level and your training age.
Go in easily if you have never trained in such high volume.