Calculating drift/precision on a temperature circuit
Your system equation is OUT= G2 * (G1*RL - 2.5*R1/(R1+R2)) 

you need 

delta(OUT)/delta(t) ~~ d(OUT)/d(t) = d(G2)/dt *(G1*RL - 2.5*R1/(R1+R2)) + G2 * d(G1*RL - 2.5*R1/(R1+R2))/dt

where t is tempereture.( d(G2)/dt equal to drift G2 amplifier) 

the first element is the effect of drift of G2 in that bias point, the second is the effect of other drift with gain G2. ...

you can assign d(OUT)/d(t) ~ delta(OUT)/delta(t)