OK. It's hard to describe the extent to which you are not getting it.
Don't imagine the team you're picking won't be going up against some Test-standard outfit. That is pointless. Imagine instead that you are a selector some level higher than Test standard. You might not be superior to this hypothetical opposition. Imagine that your opposition is a fictional side that is the best opposition whether that's possible or not.
So now, the pick between Gilly & Knott is to do with tactics. It's OK if you want to pick Knott to have for keeping skills. But you can't assume your top 6 will beat all opponents because we don't know who the opposition is.
it is simple, dude.
my top six has to be the best top 6 possible. ever. i am not going to bring in a batsman who can bowl a bit in place of a better batsman and compromise on my batting. i will select six batters who can score runs anywhere against any combination. this top is the best six batters possible. you can bring a lara in place of sachin or a hammond in place of richards. but the combination still carries similar weight because this is as good as it gets, really.
my bowling combination is the best bowling combination. ever. i am not going to bring in a slightly inferior bowler who can score a few more runs and spoil my chances of taking 20 wickets for minimum runs on any kind of surface against any kind of opposition. my four bowlers are the best four bowlers i can put together as a bowling unit, not just as individual bowlers. lillee, marshall, warne and akram (and sobers) are the best combo possible. replace one or two of them with a barnes or a hadlee or a murali, it doesnt change much. this is the best combo possible.
i am going to choose the best wicket keeper of all time. i dont care if he scores a few runs less than a slightly inferior keeper as long as my chosen keeper can hold on to every chance that comes his way any time of the day. i think gilly is fine for the job.
but chappell thinks knott is the best keeper he has seen. so he doesnt care that gilly scores a few extra runs. it is more important for him that, he believes, knott will take all the chances that come his way whereas gilly might spill a catch or two.
if you think gilly is as good a keeper as knott was, like i do, then choose him. but deep down if there is a doubt that he is slightly suspect then go for knott. there is nothing wrong in chappell asking you to ignore gilly's batting ability and go only for pure wicket keeping skills. a catch can be worth more than 15 extra runs gilly can score over knott, you know?