"I've heard someone claim that the Gaels of Scotland oppressed the Picts,
and that is why there is very little information about them."

Scotland north of the Forth in the middle of the first millenium was peopled by the Picts and the Gaelic Scotti of Dalriada. The Picts would have been far more numerous and the Pictish area was far bigger as Scottish Dalriada was basically just Argyll and some of the Hebrides. Slightly later due to conflict Dalriada was totally occupied by the Picts and several of the Pictish Kings were also described as Kings of Dalriada. Both entities then came under pressure from Norse invaders as the islands (both Hebrides and Northern Isles) were completely lost and it is also thought that perhaps many of the Pictish nobility perished in conflicts with the Norse. There seems to have been a coming together of the Scotti and the Pictish nobility and it is thought many of the latter were possibly already at least partly Gaelicised through the influence of the Celtic Church. Eventually Kenneth Macalpin (either of Scotti or at least part Scotti descent) took the throne (either by succeeding to it or by violence)though contemporary sources still describe him and his immediate succesors as Kings of Pictland rather than Kings of Alba (ie Scotland). When we enter the real more written historic period of the second millenium the Pictish language has either disappeared or at least was disappearing. There is no evidence that the Pictish people themsleves were killed, removed or anything else. Rather the new Kingdom of Alba (later Scotland) simply took on a Gaelic identity.

Later Scottish propaganda also played down the Pictish (ie British) factor. For instance the Declaration of Arbroath insists that the Picts were driven out. This was probably because the English monarchy was presenting itslef as the new Bretwaldas (ie High Kings of Britain) and successors to the idea of Arthur etc - so the Scots were saying 'well that doesn't include us'.

Of course most Scots in the former Pictish areas were descendents of the Pictish peoples who themselves were simply the descendents of those tribes of Britons who remained outwith Roman Britannia. The Pictish language may have disappeared but the Pictish symbols (eg St Andrews as patron saint and the Saltire) did not and also of course the centre of Scottish power (at least until it exapnded south)was very much centred on the area of the southern Picts.

The Britons of southern Scotland have been wiped from history far more thoroughly than the Picts were though!