It means Walcott will score more runs than Gilchrist and even if he drops an extra catch he can make up for it by the runs he does score additionally when compared to Gilchrist. I realize you are having a hard time as I am making it sound more complex than it is. Walcott kept only early in his career and then later on, he played as a batsman. And from every account, before his back injury, he was an excellent keeper and was kept in the side for his keeping even his batting was not great initially. As he established himself more as a batsman, he unfortunately had the injury and had to give up the gloves. But he was an excellent slip fielder even after that and again, by all accounts, if he was fit, would have still been a very good wicket keeper.
I won't pick Walcott to keep even for my Windies XI, forget a world XI. But Milenko has. And when I am comparing the two, I am thinking of who will have the bigger impact on the game as a whole, coz we are not talking about a statistical exercise here, but an actual cricket match. If you wanna assume Walcott would have averaged only 40 had he kept, then what you are saying will make sense. I don't. Its the Sangakkara argument. Just because they gave up as they started getting into their batting prime, does not mean they would not have had those runs had they been keeping. That is at least my take on it and why I rated Walcott over Gilly in this comparison. If you are on the other side of the argument, I understand. Its not a given here but for my argument, the evidence is always AB De Villiers. He gave up keeping, became an ATG batsman and took back to keeping and was still an absolute gun of a batsman. To a lesser extent, it was also proved by Dravid in ODIs.
Unlike a couple of my other comparisons here, I am not gonna harp on this comparison. I know its a controversial take but it is what I genuinely feel. But I realize why others may not feel this way and that's fine.