The Republican Monument has been studied in detail by Sole. It is situated to the south of the Decumanus, opposite the theatre, between the east gate of the Castrum and the Porta Romana, that forms part of the city wall from the first century BC, probably the work of M. Tullius Cicero.
What remains of the monument is a square base (6.10 x 6.10) of large tufa blocks, with a cornice at the bottom. Three layers have been preserved, but Vaglieri saw traces of another five layers, which means that the base was at least 3.80 high. From a typological point of view the cornice seems to belong to the period c. 150-100 BC. The monument is at a low level, 1.40 below the present level of the Decumanus and 0.70 above the sand. The following layers are documented between the Castrum and the Porta Romana:
- A raising of c. 0.70 on top of the sand. On this layer rest the Republican Monument and the Cippi of Caninius, that have been dated by Meiggs to c. 150-125 BC.
- A raising of c. 0.80 on top of layer 1. On this layer rests the Republican Temple on the corner of Via dei Molini. This raising should probably be dated to the reign of Claudius.
- A raising of c. 0.l5 on top of layer 2, probably from the reign of Domitian.
- A raising of c. 0.25-0.75 on top of layer 3, probably from the reign of Hadrian.
- A final, slight raising, probably Severan, taking the final level to c. 2.00-2.50 above the sand.
A comparison with similar monuments suggests a date of the monument in the period 120/110 - 70/60 BC, and according to Sole it is most likely to be assigned to the first century BC. In trenches in this part of town countless remains were found of buildings from the second and first century BC, but other tombs were not found. This is reminiscent of the situation outside the Porta Marina, where only two large tombs of leading Ostian citizens have been found. In the second century AD buildings were erected to the east, south and west of the monument. It remained visible from the north, from the Decumanus.
According to Coarelli the monument was a tomb from the late first century BC, of Gamala, son of the builder of the Quattro Tempietti to the west of the theatre. He suggests that there is a relation with the Horrea of Hortensius to the east, and that this store building was built by Gamala. An objection to Coarelli's hypothesis is, that the funerary monument would then have been built inside the city wall, which is abnormal. According to Zevi it was a funerary monument with an aedicula or altar on top, from the first half of the first century BC. The funeral chamber must be in or below the base.