Both GWAS and eQTLs often have allelic heterogeneity - multiple independent loci close to each other influence the same trait/gene. You can use this information as a natural experiment to infer causality of a gene in trait levels. Do slight wiggles in levels of a gene expression lead to slight wiggles in the level of a trait? If so, that’s evidence for causal function of the gene in the levels of the trait. Led by Mike Love, we developed a tool called MRLocus that integrates eQTL and GWAS to identify causal genes mediating trait differences. Read more about it here: https://www.biorxiv.org/content/10.1101/2020.08.14.250720v2.full.