Is there any reliable source that explains the Tsugumi Ohba-Hiroshi Gamo pen name?
These are still just rumors and speculation.

The most notable are in Death Note and Bakuman.

In Death Note, the seminar Light goes to is called "Gamo Seminar":



In Bakuman, there is reference to a series called Super Hero Legend. Not only is the concept similar, but even the drawings of this are very clear references to the art style of Hiroshi Gamo's 1993 work Tottemo! Luckyman.