Gamma more than 2 can simply mean that you have chosen a wrong maximum for Tm. This is possible if you have two peaks (instead of one) at T=Tm1 and T=Tm2. Let us assume that Tm2>Tm1. For each peak we have related Gamma. Namely, 

Gamma1=Ln(1/Er-1/Em1)/Ln(T-Tm1)

and 

Gamma2=Ln(1/Er-1/Em2)/Ln(T-Tm2) 

where Em1 (Em2) is the maximum value of Er at Tm1 (Tm2). 

Most likely (with good accuracy) we can assume that Em2=Em1 (or very close). In this case we get a rather simple approximate relation between the two Gammas:

Gamma2=Gamma1*{Ln(T-Tm1)/Ln(T-Tm2)}

Since for Tm2>Tm1, always Ln(T-Tm2)Gamma1. Therefore, even for the largest value of Gamma1 (=2), we  will have Gamma2>2.