If you want to say 即英语好是GMAT考了高的必要条件, then you should claim: Since my GMAT score is high, my English is superb. This is different from the following statement: Since my English is superb, my GMAT score is high.
LZ, you confused the definition of necessary condition and sufficient condition.
(1)我GMAT考了高,因为我英语好 (2)我英语好,所以我GMAT考了高
In both statement, the sufficient condition is My English is good; and the necessary condition is My GMAT score is high. Because leads a sufficient condition; therefore leads a necessary condition.
If the premise and the conclusion in two different statement are the same, but their relative positions in the argument are different, they are still the same logical statement.
I love you very much because you have a beautiful mind. B --> L Since you have a beautiful ming, I love you to the bone. B --> L