Gwalior was one of six so-called Convention States, which entered an agreement with the Indian PO around 1885, to overprint their State names onto Indian stamps.
Gwalior was the biggest and most prominent of the six, and the only one permitted to use a non-English overprint. Its Service (government use) stamps were overprinted entirely in Hindi

(SG O69)
although it's general use stamps showed GWALIOR in both Hindi and English:

(SG 134)