There are multiple lakes named Gem lake. This is the lake at the end of Little Lakes Valley to Gem Lakes trail. This trail is one of the more popular day hikes near Mammoth Lakes. The trailhead starts near Tom’s place and parking gets full during long weekends rather quickly. At the end of this stunning trail are Gem Lakes.
Even in September, there was still some snow left along the lakes. This is also a popular beginner backpacking campground, a bit too crowded for my liking though.
The trailhead can be found in the AllTrails link (above) or the USDA website.