Re: Issues with Grubee GT2 Frame
Many of the photos on the Grubee web site are old, very old. Years back the GT2 frame from Grubee did have a threaded screw on gas cap. Not so anymore (as best I can tell). Now they all use the "quarter turn" gas cap the same as what comes with the engine kit 1.5 l gas tank. If it is hard to install spray it (on the underside, to include the rubber gaskets) with WD40.
As to the built-in frame tank fuel outlet, mine was threaded to accept the standard bike engine kit supplied pet cock. It was a wee bit difficult to get started but it fit and does not leak.
BTW, some here will tell you that your frame is not a Grubee frame even though the Grubee frame may look identical to your frame. My suggestion? Ignore them who say such.