I just learned that the author commented his script (so it's here, but in comment = don't show up)
what you can do is to create a ".gmic" file or "user.gmic" depending your OS in your home directory and put this code:
#@gmic
#@gui _<b>Artistic</b>
#****************************************
# watercolor
#****************************************
#@gui Watercolor : fx_watercolor, fx_watercolor_preview(1)
#@gui : sep = separator(), note = note("General settings:")
#@gui : Color Intensity = float(1.8,0,4)
#@gui : Light Colors = float(0.1,0,1)
#@gui : Dark Colors = float(1,0,10)
#@gui : sep = separator(), note = note("Line art settings:")
#@gui : Line Art Details = float (80,50,100)
#@gui : Line Art Smoothness = float(0,0,10)
#@gui : Line Art Strength = float(0.5,0,1)
#@gui : Line Art Style = choice("pencil","ink","comic style")
#@gui : Line Art Fade = float(0,0,100)
#@gui : sep = separator(), note = note("Color brushes settings:")
#@gui : Brush Style = choice("modern","classic","abstract","line","smooth")
#@gui : Cyan Brush Size = float(2,0,50)
#@gui : Cyan Stroke Strength = float(0,-5,5)
#@gui : Cyan Edge Distance = float(2,0,10)
#@gui : Cyan Abstraction = float(4,0,10)
#@gui : Magenta Brush Size = float(2,0,50)
#@gui : Magenta Stroke Strength = float(0,-5,5)
#@gui : Magenta Edge Distance = float(3,0,10)
#@gui : Magenta Abstraction = float(3,0,10)
#@gui : Yellow Brush Size = float(2,0,50)
#@gui : Yellow Stroke Strength = float(0,-5,5)
#@gui : Yellow Edge Distance = float(4,0,10)
#@gui : Yellow Abstraction = float(2,0,10)
#@gui : sep = separator(), note = note("Paper and ambience settings:")
#@gui : Paper Color Black = bool(0)
#@gui : Add Paper Texture = bool(0)
#@gui : Invert Colors = bool(0)
#@gui : Ambient Light = float(0,-50,50)
#@gui : sep = separator(), Preview type = choice("Full","Forward horizontal","Forward vertical","Backward horizontal","Backward vertical","Duplicate top","Duplicate left","Duplicate bottom","Duplicate right","Duplicate horizontal","Duplicate vertical","Checkered","Checkered inverse")
#@gui : sep = separator(), note = note("<small>Authors : <i>Tom Keil and Arto Huotari </i> Latest update: <i>2011/19/02</i>.</small>")
#@gui : note = link("Tutorial download","http://www.slideshare.net/ArtoHuotari/gmig-water-color-filter-tutorial")
#@gui : note = link("Video demonstration here","http://vimeo.com/19713850")
fx_watercolor :
repeat $! l[$>] split_opacity l[0]
#secure details:
if $7==0 +fx_gradient_norm $5,0.5,{99.99-$4},$4,1,0
elif $7==1 +fx_edges {$5/3.5},{95-$4},0,0
elif $7==2 +fx_edges {$5/3.5},{95-$4},0,0
+fx_mix_hsv[0] 1,0,0,1,-1,0,1,0,0,0,2,0
tk_fx_channel_processing[2] 1,1,0,{$5/10},4,{{$4-50}*2},100,256,0,0,0,2,7,0
reverse[1,2] fx_compose_multiply[1,2] {0.5+{$6/2}}
fi fx_frame_round[1] 5,$8,15,0,255,255,255,255,0,0.1,3
#image preparation:
fx_smooth_anisotropic[0]
60,0.16,{{20/50}+0.6},{{20/9}+0.6},2.35,0.8,30,2,0,1,1,0,1
[0] +negate[0] move[1] 4
fx_gaussian_blur[2] {{w+h}/100},0,0,1,0,0,0
fx_compose_dodge[1,2] 1
fx_mix_lab[0] 1,0,0,$1,0,0,$1,0,0,0,2,0
reverse[0,1]
fx_compose_colorburn[0,1] $2
+luminance[0] reverse[1,2]
tk_fx_channel_processing[1] 1,1,0,0,4,{$3*5},100,256,0,0,0,2,0,0
fx_gaussian_blur[1] $3,0,0,1,0,0,0
tk_fx_channel_processing[1] 1,1,0,0,4,0,99,256,0,1,0,2,0,0
tk_fx_replace_color[1] 1,0,255,255,255,255,0,0,0,0
fx_spread[1] {$3*2},{$3*2},0,0
fx_gaussian_blur[1] {{w+h}/1000},0,0,1,0,0,0
reverse[0,1] fx_compose_multiply[0,1] 1
#channel and mask creation:
split[0] c
if $9==0 +negate[0] +negate[1] +negate[2]
elif $9==1 +fill_color[0] 255,255,255 +fill_color[0] 255,255,255
+fill_color[0] 255,255,255
elif $9==2 +fill_color[0] 255,255,255 +fill_color[0] 255,255,255
+fill_color[0] 255,255,255
elif $9==3 +fx_edges[0] {$10/5},{$13*5},1,0
+fx_edges[1] {$14/5},{$17*5},1,0 +fx_edges[2] {$18/5},{$21*5},1,0
elif $9==4 +negate[0] +negate[1] +negate[2] fi
#channel and mask processing:
tk_fx_channel_processing[0] 1,{{$11/5}+1},{$11/50},0,0,0,100,256,0,0,0,2,0,0
if $9!=2
fx_gaussian_blur[0] $10,0,0,1,0,0,0
else cubism[0] 300,{$13*5},{$13*36},1,{$10/50} fi
if $9==0
tk_fx_channel_processing... 1,{{$11/5}+1},{$11/50},0,0,0,100,256,0,0,0,2,0,0
cubism... 300,{$13*5},{$13*36},1,{$10/50} fi
if $9==4
tk_fx_channel_processing... 1,{{$11/5}+1},{$11/50},0,0,0,100,256,0,0,0,2,0,0
fx_painting... $13,1.5,2 fi
fx_frame_fuzzy... {$12*5},{$12*5},{$13*20},{$10/2},0,0,0,255
tk_fx_channel_processing[1] 1,{{$15/5}+1},{$15/50},0,0,0,100,256,0,0,0,2,0,0
if $9!=2
fx_gaussian_blur[1] $14,0,0,1,0,0,0
else cubism[1] 300,{$17*5},{$17*36},1,{$14/50} fi
if $9==0
tk_fx_channel_processing.. 1,{{$15/5}+1},{$15/50},0,0,0,100,256,0,0,0,2,0,0
cubism.. 300,{$17*5},{$17*36},1,{$14/50} fi
if $9==4
tk_fx_channel_processing.. 1,{{$15/5}+1},{$15/50},0,0,0,100,256,0,0,0,2,0,0
fx_painting.. $17,1.5,2 fi
fx_frame_fuzzy.. {$16*5},{$16*5},{$17*20},{$14/2},0,0,0,255
tk_fx_channel_processing[2] 1,{{$19/5}+1},{$19/50},0,0,0,100,256,0,0,0,2,0,0
if $9!=2
fx_gaussian_blur[2] $18,0,0,1,0,0,0
else cubism[2] 300,{$21*5},{$21*36},1,{$18/50} fi
if $9==0
tk_fx_channel_processing. 1,{{$19/5}+1},{$19/50},0,0,0,100,256,0,0,0,2,0,0
cubism. 300,{$21*5},{$21*36},1,{$18/50} fi
if $9==4
tk_fx_channel_processing. 1,{{$19/5}+1},{$19/50},0,0,0,100,256,0,0,0,2,0,0
fx_painting. $21,1.5,2 fi
fx_frame_fuzzy. {$20*5},{$20*5},{$21*20},{$18/2},0,0,0,255
to_gray. to_gray.. to_gray... to_rgba[0] to_rgba[1] to_rgba[2]
#recombine image:
split[0] c reverse[3,-3] compose_multiply[3,-3] append[0,1,2,3] c
split[1] c reverse[4,-2] compose_multiply[4,-2] append[1,2,3,4] c
split[2] c reverse[5,-1] compose_multiply[5,-1] append[2,3,4,5] c
if $22==0 +fill_color[0] 255,255,255 else +fill_color[0] 0 fi
reverse[0,-1] compose_rgba[0,-1]
if $22==0 +fill_color[1] 255,255,255 else +fill_color[1] 0 fi
reverse[1,-1] compose_rgba[1,-1]
if $22==0 +fill_color[2] 255,255,255 else +fill_color[2] 0 fi
reverse[2,-1] compose_rgba[2,-1]
to_gray[0] to_gray[1] to_gray[2] append[0,1,2] c
if $23==1 fx_paper 0,0 fi
if $24==1 negate[0] fi
fx_mix_lab 1,$25,0,1,0,0,1,0,0,0,2,0
reverse[0,1] fx_compose_multiply[0,1] $6
endl a c endl done
fx_watercolor_preview :
gui_split_preview "fx_watercolor ${1--2}",$-1
#*****END OF FILTER**********************
and um... I won't rewrite thing as it's all there >
BTW if you have an alpha channel on your image remove it before launching G'MIC, or this filter will tell you that you have too much channel