The reason you don't qualify for the Silver Slugger is due to the way the game code determines eligibility, which doesn't really adhere to the real process in MLB. You have to have a minimum number of plate appearances in the DH position since the only hitting position a two-way player can win SS for is as the DH. As a two-way player SP/LF for example, you won't get enough PA's as the DH since you are only DH three games out of every five. Since you are the starting pitcher on a five man rotation, there is a five game cycle to your positions.
You play two games as the LF and three as the DH for each of these five game cycles. In order to qualify for the DH Silver Slugger, you have to essentially play four games every five to attain enough PA's over the course of the regular season. And if you try to compete for the LF Silver Slugger say in Franchise where you can control the roster, it won't work even if you play in the field four games out of five since the game code relegates your two-way player to DH for all hitting award decisions.
The reason why just one fielding error cost you the Gold Glove is because you compete in the starting pitcher category, where nearly the entire league's starting pitchers go the entire regular season without making an error. Obviously, this is not a fair contest if you are playing around 64 games in left field for example. But, that's the way the game code determines it. The first criteria for GG is errors. The other factors like put outs and assists are used as tie breakers, which again for pitchers is really the determining factor. By playing in the field, you rack up far more outs and assists than a dedicated starting pitcher. So, if you can manage to avoid an error, you get the GG.
