to try and make things easier to understand
first of all G.M vehicles are renowned for the dpf to be placed further away in the exhaust system from the manifold than other manufactures so more heat needs to be generated at the dpf to give a successful regeneration. Although as with any modern diesel local runs where the vehicle only just reaches operating temperature will give issues fact, to try and compensate for this manufactures have and continually bring software updates out to try and compensate for this. the fact remains there will always be problems especially on vehicles use for school runs supermarket trips etc. Other issues are the EGR.
regeneration happens when the ecu tells the car to do so approx. every 300miles. When the happens the cars timing is altered slightly to allow extra fuel to be inserted, this will then enter the exhaust system which will burn and help in a regeneration of the dpf (soot burnt off). Great if your on a A road or motorway as the temperature within the exhaust will allow all this to happen over a set period of time. The problem arises if you are in a start stop situation such as around town when one takes place, if this happens the ecu will stop the regeneration process this will take place around 3 times if no successful is taken place after that it will not regenerate till the next given time. The problem if this was to happen the dpf gets fuller and fuller, the other all the extra unburnt fuel has to go somewhere and it ends up in the sump which will dilute the oil (losing its protection properties) and if its left to get so high theres a risk the engine will rev its bollocks off until destruction.
passive regeneration is when your going up and down the motorway say for a prolonged time the temperature with in the exhaust will become hot enough to partly cause the catalytic effect within the dpf to burn off some of the soot but not enough to fully clean hence above, kinda keeps things ticking over until that time.
forced regeneration is when the dpf is so full/blocked it requires manufacturer intervention to use diagnostic equipment to force a regeneration. criteria's have to be met before it will be done (no other faults) the diagnostic equipment will rev the engine up to a point of destruction lol and hold the revs there over a preset time and then bring them back normal over a time. This can or cannot be successful.
The 2500rpm you refer to is actually 2250rpm min hence 2500rpm or above travelling down say a motorway in 4th or 5th gear to keep speed in toe enabling control of the speed and hopefully a non stop journey or allowing the revs to fall below the preset given above for 20min or more depending on how blocked it is. This will cause the temperatures in the exhaust to become that high that you are trying to mimic a forced regeneration you could say. the engine will be under load and the egr will be shut as with the ecu based regeneration type. This can be very effective and often works although not quite as good as a forced one as the vehicle will be moving and not stationary hence more airflow (cooling).
additives are used to lower the temperature in which a regeneration will take place causing a higher burning temperature in the normal engine cycle. there good and bad ones out there that accomplish this. Archoil ar6400 is good and BG244. it also has to be said the way it is used will also have an overhaul effect on the way it does it. some companies will injexcrt it directly into the system rather than diluting it in the fuel.......its can be better sometimes to apply a high dose if diluting in fuel though read manufacturers instructions first