Glycosylation is the most frequent and structurally complex posttranslational modification in cell-surface and secreted proteins. Glycans are major orchestrators of biological processes, namely, by controlling protein folding and key biological functions such as cell adhesion, migration, signaling and immune recognition. Altered glycosylation is considered a hallmark of malignant transformations that decisively contributes to disease outcome. This review comprehensively summarizes the main findings related with gastrointestinal cancers and the decisive impact of aberrant glycosylation on tumor biology toward more aggressive phenotypes. Particular emphasis is given to alterations in O-glycosylation, namely, the overexpression of immature O-glycans, and the sialylated Lewis antigens sialyl-LeA and sialyl-LeX, frequently implicated in lymphohematogenous metastasis. We further discuss how recent contributions from glycoproteomics and glycoengineering fields have broadened our understanding of the human O-glycoproteome and its implications for cancer research. Finally, we address the tremendous potential of glycans in the context of targeted therapeutics (selective inhibition of glycosylation pathways, immunotherapy) and discuss the need to include glycomics/glycoproteomics in holistic panomics models toward true precision medicine settings.