G̃ / g̃ is a letter which combines the common letter G with a tilde.
The letter does not exist in many alphabets. Examples of alphabets with this letter are:
Unicode encodes g with tilde with a combining diacritical mark (U+0303 ̃ combining tilde), rather than a precomposed character. As such, the tilde may not align properly with some typefaces and systems. Additionally, owing to the difficulties in inputting this character, Guarani speakers often replace it with g with circumflex (ĝ) or omit the diacritic altogether.
Most Guarani speakers don't use this character, instead spelling this sound the same as a plain g.