what’s the difference between GPS altitude and pressure altitude.

Technically, GPS altitude is accurate but not precise.

Barometric altitude is precise but not accurate unless it has been calibrated. 

If you are curious about difference between accuracy and precision you can google that.

I put them side by side as datafields within a data page in the same GPS activity,they are usually pretty close to each other in measurement handy if your worried if the watch sensor might be off,although I'm not sure as to which is more accurate. 

None is accurate enough to be accurate all the time. Give it up or you'll go crazy. Żaden nie jest dokładny na tyle żeby był cały czas dokładny. Odpuść bo zwariujesz.