A limited number of experimental studies have been carried out on the application of prestressing to glass FRP bars. Traditional anchorage systems designed for steel are not suitable for FRP. In this study, metal bonding anchorages were produced for post-tensioning application in GFRP bars. These anchors consist of metal tubes that hold the bars, which are then filled with epoxy resin. In this system, each bar has a dead end and a live end. A piece with one conventional 7-wire prestressing strand is threaded on an adapter at the live end anchorage. The prestressing force is applied to the steel with traditional prestressing equipment and transmitted to the GFRP bar. An external nut is used in the live end anchorage to lock the prestressing force. Two sets of anchorages were evaluated, with pipe lengths of 10 cm and 30 cm. The shorter length resulted in slippage of the bar within the anchor, leading to significant prestressing losses after 118 h of monitoring. Additionally, the live end anchorage failed on one of the bars. In contrast, the anchorages with a pipe length of 30 cm showed minimal prestressing losses. These larger anchorages proved to be effective in applying and maintaining the prestressing force in unbonded prestressed GFRP bars.