available_terminals = __gnuplot_get_var__ (gcf, "GPVAL_TERMINALS")
For me I get ...
available_terminals = aqua canvas cgm corel dpu414 dumb dxf eepic emf emtex epslatex epson_180dpi epson_60dpi epson_lx800 fig gif gnugraph gpic hp2623A hp2648 hp500c hpdj hpgl hpljii hppj imagen jpeg latex lua mf mif mp nec_cp6 okidata pbm pcl5 pdf pdfcairo png pngcairo postscript pslatex pstex pstricks qms regis rgip starc svg tandy_60dpi tek40xx tek410x texdraw tgif tikz tkcanvas tpic uniplex unknown vttek x11 xlib xterm
Alternatively you can try the command below from gnuplot
gnuplot> set print GPVAL_TERMINALS
aqua canvas cgm corel dpu414 dumb dxf eepic emf emtex epslatex epson_180dpi epson_60dpi epson_lx800 fig gif gnugraph gpic hp2623A hp2648 hp500c hpdj hpgl hpljii hppj imagen jpeg latex lua mf mif mp nec_cp6 okidata pbm pcl5 pdf pdfcairo png pngcairo postscript pslatex pstex pstricks qms regis rgip starc svg tandy_60dpi tek40xx tek410x texdraw tgif tikz tkcanvas tpic uniplex unknown vttek x11 xlib xterm
Ben