Human gastric lipase
There is a crystal structure of human gastric lipase (PDB ID 1hlg). The accompanying paper does not mention any quaternary structure and shows the protein as single chain. Automated analysis of the coordinates support that. The enzyme is glycosylated, so the molar mass does not match the one expected from the sequence. The apparent molar mass determined experimentally is about 50,000 g/mol, corresponding to a monomer.

However, the annotation in the structure (probably erroneously) say that the biological assembly consists of the two chains in the asymmetric unit. Also, there is a report that the size of the molecule changes with pH (90,000 g/mol at pH 6 vs 45000 g/mol at pH 3).

The same group that solved the human lipase structure later solved the dog lipase structure (in an open conformation with a bound ligand) where the annotation in the PDB deposition says it is a monomer. I doubt that there are interspecies differences between dog and human in terms of quaternary structure, so that would be another piece of evidence supporting that the human enzyme is a monomer in its native form.