The simplest way to fix or at least mitigate the problem is to tweak the gp you get from adding g12 gear. The problem is that each piece of g12 gear provides a huge bonus to the toon but only a couple of hundred GP. I think if you increased the gp from g12 gear by 10x you see some viable advantages to a broader deck vs a tall.
That said I honestly don't understand all the angst.
The collectors have bad arena ratings because they focus their resources on trash characters. Most of the ones I've seen have an arena rank in the 1,000's. If you have been ok with an arena rank in the 1,000's why do you think you should have a good shot at taking 1st place in the "Grand Arena"?
Finally because the arena is awarding prizes on each round and the losers from the first round are matched together in the 2nd round then the collectors are likely going to be competing against each other in the second round. Likewise winners from round 1 will likely be all tall decks so the arena by its nature will become more competitive and fair each round as those with competitive advantages are matched up with others of the same advantages.
Also as a side note it's been stated that the GA will have different rules periodically so the next one might be all ships. Those who have focused on ships may have the same GP as someone who avoided ships and have a clear advantage. Or they could do one where the number of defense spots is doubled giving a clear advantage to the collectors.