That's my understanding, if you are LC and are awarded a GS then the first day is GS pay- whatever that may be for you, but if I'm not mistaken you're back to non GS pay on the rest of your on-call days just like if GS a trip that goes from X-days into on-call days.

23.u.something: A long call reserve pilot who is awarded a GS rotation with a report that is within 12 hours of the first attempted contact will receive single pay, no credit for the first duty period of the rotation (in addition to any other pay and credit for the bid period).

Green Slips (GS)
A reserve pilot may submit a GS to fly on either on-call or X-days. A reserve pilot who has flown a GS rotation(s) will receive:
• single pay and credit for the portion of such rotation(s) flown on his reserve on-call days that occurred before the pilot’s accumulated credit exceeded the ALV (applied against his reserve guarantee), and
• single pay, no credit (in addition to any other form of pay and credit for the bid period) for the portion of such rotation(s) that:
• interrupted his X-day(s), or
• occurred after his accumulated credit equaled the ALV,
Note: A reserve pilot who flies a GS rotation into an X-day(s) is entitled to additional time free
of duty under Section 23 S. 11.