In the scheme of long-distance, high-altitude mountain treks, I'd call the Gokyo Lake trek "moderately difficult". The Three Passes Trek certainly has more hard days but crossing Renjo Lak was harder than anything we faced on the Everest Base Camp trek, making the most famous Nepal trek roughly similar to the Gokyo Lake trek. The Annapurna Circuit trek has its moments but is more gradual overall, and Annapurna Base Camp is fairly steep but considerably shorter than any of these. We have not hiked the Manaslu Circuit trek yet but I believe the altitude is mostly lower, although it covers a bit more distance.