The Gemara Arakhin 9b discusses certain rules of the calendar. Specifically, it mentions that Rosh Hashana will always be on the same day of the week as the day of the Omer (the 2nd day of Pesach), or the day later.
Rashi there comments that with today's fixed calendar, this is no longer in flux; rather, Rosh Hashana is always on the same day of the week as the day after the Omer (the 3rd day of Pesach). This is because the latter half of the year is always kesidran (i.e. a malei, 30 day month, always follows a chaser and vice versa), so there are exactly 161 days, which is divisible by 7, between the 3rd day of Pesach and Rosh Hashana.
Now, here's the quote from Rashi that I don't understand
האידנא עבדינן לעולם חדשי ימות החמה כסדרן ויש לנו סימן קבוע א"ת ב"ש ג"ר לשלישי של פסח הוי ר"ה
As I explained, I understand Rashi's point; I just have no idea what this א"ת ב"ש ג"ר mnemonic is.
Can any calendar experts help me out?