i agree it would be a long shot for gradkowski to see meaningful action this year... i think the original plan may even have been for him to be practice squad player... with simms, rattay & mccown being three QBs... with mccown being down that sort of opened the door... but he will likely be QB3 this season...
next year could get interesting, though... i can see a scenario where condon thinks simms is worth more than they want to pay, leading to an impasse... they could end up ahead even if they settle for less than what they want ideally by going elsewhere... if that still happens to be more than TB offers...
we'll see next year... i'm sort of simms agnostic though i may sound down on him... he did have 4 TDs in last two regular season games, & i think was something like 25-38 in playoff game against redskins (which they could easily have won but the sean taylor FR TD return was a killer)...
as to how they viewed gradkowski... again, i think he was viewed as long term developmental guy... some reasons he wasn't coveted could have been level of competition concerns (toledo?)... though MAC QBs have done well in recent years... pennington before shoulder problems, leftwich, rothlisberger, frye... he didn't have elite measureables (less than ideal height & size as i recall, may not have great speed or cannon for arm)...
but with mccown injury that timetable could be accelerated... it is possible their opinion of him could change through seeing him up close... maybe they did undervalue & underappreciate him based on film, but that could be subject to change if he impresses more in person... things like how well & quickly he picks up a pro offense may not have been evident from highlights... players also have variable rates of developement, capacity for taking to instruction, learning, understanding...
before gradkowski had been coached up by gruden... he hadn't yet been coached up by gruden...

its possible he didn't look as good with lesser coaching... but if he responds well to gruden, shows a tendency to pick things up very quickly... these are some things that may not have been as apparent beforehand (causing him to be undervalued, by TB as well as everybody else) but could impress gruden AFTER the scouting evaluation process proper...
while not common, we wouldn't have to rack our brains too hard to think of day two QBs that excelled & far outperformed their draft slot-based expectations, & did far better than many QBs of higher pedigree with better measurables taken ahead of them...
one of the most interesting things about football is that it is so refractory to being boiled down to a science... it is as much art... big case in point is how intangibles, mental & character & personality traits (& thousand other things) can make or break a QBs career... will a QB panic when under withering pressure, or remain poised & composed? that is hard to tell from 7 on 7 drills, or even to project from college to pro...
the disparity in talent, speed, strength, athleticism of the defenders is no doubt huge (as is the complexity of pro offenses & defenses)... but i think that whatever the differences are, they are differences that play a critical difference in whether QBs make it or not... we have all seen many instances where QBs that had legendary prep exploits & storied college career... only to fall completely flat on their face at the last level... very hard sometimes (almost impossible in some cases) to tell in advance for which players the challenges presented by NFL game will be a threshold they are incapable of getting across and which proves insurmountable... and those that have "the right stuff" (whatever that is)...
the problem may extend further than scouts having imperfect understanding of the most important traits needed for QB to succeed... there is no one bluprint for success... many traits are needed, & being poor in some (if not too devestating) can be compensated for & overcome on a net, overall basis (not easy to categorize a kaleidoscope of potential & alternate psycholical profiles... i think the brain doctor dude may already be doing something like this, but the equivalent of a meirs-briggs typology would be better than nothing... it would be interesting to look at QBs with that lens (not just in a few isolated cases but wide scale)... ANY lens... from which a systematic body of thought could be built up on a principled basis...
i guess i have taken a lot of words to say that a good deal of why some QBs succeed & most fail has to do with, for lack of a better word, psychology... & that is a science not quite on a level of exactness as math, science, engineering...
a further issue would be, even if there was some kind of magical checklist scouts could tick off in the abstract... there might still remain the problem of IDENTIFYING which QB prospects had those traits, to what degree, how to measure (some traits elude written tests & even sophisticated meiers-briggs typologies)?