how to generate a function using symbolic expression and plot that

I'm trying to calculate the root,gamma of my function over a range of angles from 0 to 90 but it's giving om errors which I don't know how to fix. here is my code and the error message,

function [g1] = rtfnd(n_1,nu_bar_0_cm,thetad)

im = sqrt(-1);
c = 2.99792458e+08; 
mu_0=4*pi*1e-7;
eps_0 = 8.8541878176e-12; 
phid = 0;
 nu_bar_0 = nu_bar_0_cm * 1e2; 

lambda_0 = 1/nu_bar_0;
eps_1 = ((n_1)^2)*eps_0*eye(3);
nu = c/lambda_0;
lambda_1 = lambda_0/n_1;
 w=2*pi*nu; 
 
 Al = 2*pi*sind(thetad)*cosd(phid)/lambda_1;%alpha
 Bt= 2*pi* sind(thetad) * sind(phid)/lambda_1; % beta
 
syms gamma 
x11 = (w^2)*mu_0*eps_1(1,1)-(Bt^2)-(gamma^2);
x12 = (w^2)*mu_0*eps_1(1,2)+Al*Bt;
x13 = (w^2)*mu_0*eps_1(1,3)+Al*gamma;
x21 = (w^2)*mu_0*eps_1(2,1)+Al*Bt;
x22 = (w^2)*mu_0*eps_1(2,2)-(Al^2)-(gamma^2);
x23 = (w^2)*mu_0*eps_1(2,3)+Bt*gamma;
x31 = (w^2)*mu_0*eps_1(3,1)+Al*gamma;
x32 = (w^2)*mu_0*eps_1(3,2)+Bt*gamma;
x33 = (w^2)*mu_0*eps_1(3,3)-(Bt^2)-(Al^2);

M = [x11 x12 x13; x21 x22 x23; x31 x32 x33];
detM = det(M);
gamma = solve(detM,gamma);
g1 = double(gamma);

%%%%%%%%%%%%%%%%%%%%% i called this file testroot_2.m in the workspace
n_1 = 1 ;
    nu_bar_0_cm=1000;
    theta_points = 361;
    theta_start = 0;
    theta_end = 90;
    theta_incr = (theta_end - theta_start)/(theta_points - 1);
    theta = theta_start:theta_incr:theta_end;
     
      
     for ii = 1:length(theta)
         
         A = rtfnd(n_1,nu_bar_0_cm,thetad);
     end
     %%%%%%%%%%%%% this is the error message
     >> testroot_2
Error: File: rtfnd.m Line: 49 Column: 1
"gamma" previously appeared to be used as a function or command,
conflicting with its use here as the name of a variable.
A possible cause of this error is that you forgot to initialize
the variable, or you have initialized it implicitly using load
or eval.


Error in testroot_2 (line 15)
         A = rtfnd(n_1,nu_bar_0_cm,thetad);

Answer 1

[Walter Roberson]

Replace

syms gamma

With

gamma = sym('gamma');