Not sure about your opponents not being able to kill a DR team, i suppose that the overall GP anyways doesnt tell anything at all about a roster. Lower divisions earlier tend to be usually a lot easier (mods, low amount of pure g13).
However top3 for GAC are to 95%+? pure GAC rosters - i.e. those have no KAM R7+ clones, no other unneeded relic for TBs and are just maden slim regarding any unneeded skill/omega/zeta wherever possible.
And in the end TB/Kam players play against artificial tuned down GAC rosters. Its just another paper zombie way of tuning your roster downwards to gain benefit. Thats the reality. You talk often about a good roster management, but in the end its more of tuning down the most efficient way. This type of game basically is a collectors game and you should upgrade, but GAC punishes exactly this behaviour.