How to determine stopband of discrete Gaussian, stdev sigma, support N
I found answer at https://courseware.ee.calpoly.edu/~fdepiero/curr/dsp/dsp11.htm . Converted to the present problem, in MATLAB:

kernel2 = fspecial('gaussian', [1 5], 1.3)
spike = [1 0 0 0 0];   
[h, w] = freqz(kernel2, spike, 1024);
m = abs(h)
m_db = 10*log10(m/max(m));
plot(w,m_db)


Because the compact support Gaussian had zero crossings, I used the first zero to define the stop band.