This issue as to whether glass may be Kashered is not dealt with in the Gemara. It is, however, a matter of dispute among the Rishonim. In order to grasp the basis for their controversy, we need to first survey how the Talmud regards glass in areas other than Kashering.
The primary source for this topic is Avos D'Rabbi Natan 41:b which states that "glass utensils do not absorb or exude." Although this statement does not appear in the context of Kashering, it nevertheless teaches that glass utensils differ significantly from metal and earthenware utensils. The Torah unambiguously indicates (Vayikra 6:21 and Bamidbar 31:21-23) that metal and earthenware utensils absorb and retain food particles from the hot food items that are placed in them.
This source, however, does not simply resolve the question regarding the status of glass in the context of Kashering. This is because Chazal assign the status of metal and earthenware to glass in the areas of "Tevilat Keilim" and "Tumah and Tahara." Moreover, regarding the area of "Tumah and Tahara" Chazal assigned glass the status of earthenware.
The Gemara in Avoda Zara 75b states the Chazal decreed that glass utensils need to be immersed in a Mikvah prior to use, if it had previously been owned by non-Jews. Even though the Torah only required metal utensils to be immersed, Chazal decreed that glass utensils are also required to be immersed. This is because, the Gemara states, glass shares a common property with metal. Namely, that if they are broken they can be fixed. Rashi explains that both glass and metal are repaired in an identical manner, they are both melted down and refashioned.
Regarding the area of "Tumah and Tahara," the Mishna (Keilim 30:1) states that glass can contract "Tumah" despite the fact that there is no allusion to this phenomenon in the Torah. The Gemara (Shabbat 15b) explains that Chazal decreed that glass can become "Tameh." The reason given is that glass shares a property with pottery, both are from the ground.
While these two rabbinic decrees appear to be contradictory, the Meiri to Shabbat 15b explains that they are not. He explains that essentially glass is comparable both to metal and earthenware. Hence, the Gemara imposes the stringencies of metal and earthenware upon glass.
Rishonim
Three opinions appear in the Rishonim regarding the status of glass as far as Kashering is concerned. Tosafos (Avoda Zara 33b s.v. Koonya) appears to rule that glass does not absorb food particles and need not be Kashered if it absorbed non-Kosher or chametz food particles. A number of Rishonim including the Rashba (Teshuvos, 233) agree to this ruling. This ruling is grounded in empirical evidence and the statement regarding glass that appears in Avos D'Rabbi Natan.
The Mordechai (Pesachim 3:574) cites a ruling from Rabbeinu Yechiel of Paris that glass cannot be Kashered. Just as the Gemara imposes a stringency of earthenware on glass in the context of Tumah, Rabbeinu Yechiel believes that Chazal similarly impose the stringency that earthenware cannot be Kashered upon glass. The Terumat Hadeshen (132) rules in accordance with this opinion regarding Pesach.
The Ritva (Pesachim 30b s.v. Li'avid) cites the Ra'ah who adopts a middle approach. He rules that while we do believe that glass absorbs taste particles from food, it nevertheless may be Kashered. The Ohr Zarua (Hilchos Pesachim 25b) also rules this way even regarding Pesach laws.