The mechanism of arbuscular mycorrhizal fungi (AMF) affecting insect resistance in plants is well depicted at the physiological level. However, there is a lack of systematic research from the perspective of insects. In this study, we used AMF-Populus alba × P. berolinensis seedlings that could either increase or decrease the resistance to gypsy moth larvae, to elucidate the mechanism of mycorrhizal-induced resistance/susceptibility at the larval microbial and metabolic levels. The results showed that the larval growth was significantly inhibited in the AMF, Glomus mossae (GM) colonized seedlings, whereas enhanced in the Glomus intraradices (GI) colonized seedlings. Gut microbiome analysis elucidated that GM inoculation reduced the probiotic abundance (e.g., Staphylococcus, Lactobacillus, Akkermansia) in gypsy moth larvae and inhibited the detoxification and metabolic functions of the gut microbiota. However, GI inoculation improved the gut environment in gypsy moth larvae by reducing the abundance of pathogenic bacteria (Chryseobacterium, Fusobacterium, and Neisseria) and activating specific metabolic pathways (e.g., energy metabolism). Non-targeted metabolomics analysis further revealed that GM inoculation triggers a metabolic disorder in the larval fat body, accompanied by the down-regulation of metabolic pathways involved in detoxification and energy production. The levels of differentially accumulated metabolites related to amino acid synthesis and metabolism and exogenous toxin metabolism pathways were significantly increased in the GI group. Taken together, disturbance in the gut microbial function and fat body metabolism in gypsy moth larvae led to the induction of mycorrhizal-induced resistance after GM inoculation, and GI-induced susceptibility involved in the improvement of gut environment and energy metabolism.