The　effects　of　both　molecular　adsorption　and　external　electric　field　on　the　electronic　properties　of　germanene　are　investigated　through　first-principles　calculations　in　this　work.　The　molecular　adsorption-induced　intrinsic　electric　field　by　small　molecules　(SMs)　breaks　the　symmetry　of　two　sublattices　in　germanene,　leading　to　a　band　gap　opening　from　zero　to　3.2　meV,　even　up　to　81.0　meV.　Under　an　external　electric　field,　a　wide　ranging　linearly　tunable　band　gap　(0-69.39　meV　for　the　germanene/methane　system　and　37.66-134.17　meV　for　the　germanene/ammonia　system)　can　be　realized,　which　is　merely　determined　by　the　strength　of　the　composited　electric　field　independent　of　its　direction.　More　importantly,　the　band　gap　of　germanene/SMs　can　be　closed　and　reopened　under　a　critical　electric　field.　On　the　other　hand,　the　mechanism　of　charge　transfer　between　germanene　and　SMs　under　an　external　electric　field　is　revealed　by　an　equivalent　capacitor　model　to　explain　the　tunable　charge　transfer.　The　composited　field-induced　charge　transfer　could　be　promoted　when　an　external　electric　field　and　a　molecular　adsorption-induced　internal　electric　field　have　the　same　direction.　Otherwise,　the　charge　transfer　will　be　inhibited.　Particularly,　the　tunable　electronic　properties　of　germanene　are　sensitive　to　the　concentration　of　molecular　adsorption　under　an　electric　field,　representing　multiple-effects　that　would　significantly　enhance　the　performance　of　germanene　for　electronic　devices　in　the　future.