Spean Bridge to Rannoch Station is rideable – but you probably know that (happy to give some more info if you want).
No idea about the Gorton track – looks to be a big gap and the big flat area around NN400490 screams bogfest at me.
The rest is heresay but from people who I think are reasonably trustworthy.
The track across Rannoch Moor via White Corries lodge is apparently very hard work from a bit after White Corries Lodge until almost the road end at Rannoch, some friends did it late summer when it was supposed to be mega-dry and it took most of the day.
The third option is to go south and use Auch Gleann comes within about a km of the landrover track around Loch Lyon then the hill track over to Rannoch School from Glen Lyon – that is apparently all rideable (the Loch Lyon/ Rannoch section is used on one of the big "corporate challenge" type events up here). You can see this on Microsoft Bing maps.
Of course the Fourth Way, which we used several years ago, is to just get the train from Bridge of Orchy to Rannoch Station. 🙂