This study evaluated the effects of different levels of guanidinoacetic acid (GAA) supplementation on the transcript abundance of genes related to myogenesis, protein turnover, and energy metabolism in Nellore (Bos indicus) cattle during the growing and finishing phases. Fifty-one bulls were supplemented with 0, 3, 6, or 9 g GAA·kg⁻¹ of supplement dry matter (DM) during the growing phase and 0, 0.3, 0.6, or 0.9 g GAA·kg⁻¹ of dietary DM during the finishing phase. Longissimus thoracis samples were collected at the end of each phase for RT-qPCR analyses. During the growing phase, GAA supplementation increased the transcript abundance of genes associated with myogenic regulation (PAX7, MYF5, and MYOG) and anabolic signaling (MTOR and EIF4E), resulting in a transcriptional profile consistent with the modulation of genes involved in myogenic regulation and protein turnover, with responses differing among doses. During the finishing phase, continued GAA supplementation decreased the transcript abundance of MYF5, MYOG, EIF4E, FBXO32, and SLC2A4, whereas MTOR and GSK3B showed positive quadratic responses to increasing GAA doses. However, these molecular responses were not accompanied by improvements in growth performance, carcass characteristics, or most meat quality variables. Differences in gene expression observed at the end of the growing and finishing phases should be interpreted in the context of diet, management, feeding system, and duration of GAA exposure.