Don't recall whether have repaired a GS starter, could someone post a link to disassembled photos? Have repaired a huge number of starters over the decades (likely 100's isn't an exageration) & agree with Rockford's assessment.
One additional thought/suggestion: many of these small Japanese starters use an "O" shaped disk as the brush holder plate. The steel brush holder usually has two brush holders (some have four = two pairs), one of which is insolated from the brush holder plate while the other holder is corrected directly to the plate.
The insolated brush is connected to the "end" of the field pole windings while the other brush (the ground brush) is connected (also by its pig tail wire) to the brush holder plate. Using the conventional current desciption, current flow from the battery is as follows during starting: battery + to battery + cable to starter relay + connector, through starter relay contacts, to starter relay battery-side connector, to starter cable, to starter insolated connection & into starter, to field pole windings (note some are series while others are series-parallel), through field windings, to insolated brush, from brush to commutator, through commutator into armature windings to opposite commutator bar, through bar to ground brush, ground brush pig tail wire to steel brush holder plate, to starter frame and to ground by way of the engine case, engine case to battery ground cable and basc to battery negative to complete the circuit.
It does sound as though you have an open circuit in the starter.
The brush holder plate is clamped between the starter's steel field housing and the aluminum brush end cover. Since the brush holder plate is also cadmium plated there are many metals present in the junction. As we all know, dissimilar metals + electric current + moisture/corrosives = massive corrosion.
Typically, the connections between the brush holder and the end cap, field housing are a problem in poor starter performance so this area is always worthy of scrutiny when servicing one of these starters. I usually make a small steel "L" bracket which is bolted to the field housing and to the brush holder plate. I use star washers to assist in connection.
Others who do these starters professionally will make a ground wire with two "eye" connectors which is connected to brush holder and field housing. Same result, depends on which method one finds easier to accomplish. Either will have one using "technical language" while making the first one or two modifications but goes better after that.
This connection issue is usually only poor enough to cause high starter draw and poor starter performance, sometimes causing a no-start but can't recall one causing a complete open circuit as your seems to experience.
Rockford's suggestion that you will likely see the cause when the starter is opened shares my view but you will want to take care to inspect all possible problem areas carefully during disassembly. You may have a brush which is stuck in the holder so that its spring cannot keep it in contact with the commutator; a brush lead (pig tail) which is disconnected; the field winding joint broken loose from the head of the connector bolt which passes through the starter field housing to connect to the starter cable; an open (broken wire) in field winding or commutator; a broken/burned commutator bar; etc.
A fairly common problem is for someone to rotate the field connector bolt while connecting/disconnecting/tightening the starter cable nut. When the connector bolt is rotated, it stresses the joint connecting the bolt's head with the field windings and not much rotation is required to break the connection.
Take care when cleaning the parts as one can choose a chemical which will dissolve the lacquer insulating the field or armature windings = bad news. It will be best to simply dry brush, gently, the parts to remove dust and to wipe away oil/grease residue. Note the brush length as the brushes need to be long enough so that they bear against the commutator with good pressure. Check the brush springs and that the brushes are free to slide in the brush holders. It is amazing how often a brush will simply stick in place and open circuit a two brush starter.
What else comes to mind off hand.....when cleaning the commutator bars (the copper contact bars which connect the armature windings to the brushes), first inspect the bars carefully. If any bars are loose the armature is junk as it is not practical to have the commutator professionally replaced due to cost. If any bars are burned black, it is a sign that they have been making poor contact, if the leading edge of any bars are burned, it is likely that the preceding bar's circuit is open.
Use flint paper or 400 grit sandpaper wrapped around the commutator to clean the surface. Clamp the paper by wrapping one's index finger and thumb then rotate the paper/armature to clean the surface. Try to avoid axial sanding as it is best to have any scratches in a circumferential manner as these will cause less wear to the brushes while burnishing.
Do not use emery paper as the particles are like to embed into the copper and cut away the brushes.
The problem here is an open circuit so will dispense with attempting to describe how to identify shorts or grounds to armature or fields. You can check the field windings to ensure that there is a compete circuit between each end however this is difficult to differentiate of there is an open in only one field.
