I'm not clear on your question. the array that assembles all the G(i) is G.
Note that 3.98*10^7 is better written as 3.98e7. It faster for matlab to compute and in my opinion easier to read
Also, note that the loop is completely unnecessary:
t = 0:0.02:5;
T = 170 - 22*t;
Gbelow = 4.81e11*exp(-1500/(1.987*(T-30).*exp(-5.51e5*(T+273.15+212.273.15)./(2*(T+273.15).*(T+273.15).*(212-T)))/60;
Gabove = 3.98e7*exp(-1500/(1.987*(T-30).*exp(-2.55e5*(T+273.15+212.273.15)./(2*(T+273.15).*(T+273.15).*(212-T)))/60;
G = Gbelow;
G(T>=120) = Gabove(T>=120);
plot(t, G);