Hi, your problem sounds like the autobox is low on fluid. Not too serious...YET!
You need to check the fluid level. Now with this box that is not straightforward...
Get the car on a flat surface once the engine is warm,put in N or P, then remove fluid plug on the side of the transmission. The fluid should just reach the lower level of the hole. If not, top it up until it does. It requires FULLY SYNTH gear oil.
To be honest I would get a full oil and filter change done on the transmission. Easy (but messy) job for any garage really, should take around 1 hour (much cheaper than dealer rates!!) and will also require approx 7 litres (?) of fully synthetic transmission fluid, a filter and 2 gaskets.
Personally, if you don't know them, take the fluid and gaskets with you (available from any GM dealer) and just pay for the labour.