GAM doesn't take a duration parameter. That's what BLOCK is for: BLOCK(d) is basically GAM(4,1) integrated over a duration of 1 second. BLOCK5(d) is the same with GAM(5,1) instead -- delayed a little bit from BLOCK.

Try these commands to see what the BLOCK(1) response function looks like:

3dDeconvolve -num_stimts 1 -stim_times 1 '1D: 1' 'BLOCK(1)' -nodata 200 0.1 -polort -1
1dplot nodata.xmat.1D

To compare all 3 at once, try this:

Create file q.1D with the single line below

1*0.2 1.25*0.2 1.5*0.2 1.75*0.2 2.0*0.2

Then run the command below

3dDeconvolve -num_stimts 3 -stim_times 1 '1D: 1' 'BLOCK(1)' \
-stim_times 2 '1D: 1' 'BLOCK5(1)' \
-stim_times_AM1 3 q.1D GAM \
-nodata 200 0.1 -polort -1 -x1D stdout: | 1dplot -stdin -one -dx 0.1 -thick -thick

This will plot BLOCK(1), BLOCK5(1), and the sum of 5 GAMs spread evenly across a 1 second interval.

Then you pays your money and you takes your chances.