Metallic glasses are of permanent fundamental and technological interest due to their unique functional properties. Why does one alloy composition tend to vitrify while another one does not? What are the reasons underlying the glass formation mechanisms responsible for the existence of pinpoint compositions in metallic systems? All these questions still remain among the principal and complex issues that need to be addressed for a deeper understanding of the physical processes that determine the glass-forming ability in various alloy systems. Despite decades of intensive research, so far there are no reliable and physically substantiated methods capable of predicting narrow composition ranges of easy vitrification in multi-element alloys. In this study, we propose a phenomenological approach for predicting glass formers, which implies that a glass-forming alloy in its equilibrium crystalline state is a mixture of intermetallic phases in equal or almost equal proportions. In accordance with this strategy, we analyze glass-forming ability of a series of Gd–Co–Al alloys near the composition corresponding to the one generated by combining an equal ratio of intermetallic phases. To do that, we fabricate bulk amorphous samples by melt quenching technique into a variable-section copper mold and inspect their structure, thermal properties and critical casting size.