I mean, is the galactic credit more like paper money, such as dollars and euros, supported by national credibility? Or is it more like "metallic currency" such as gold and silver, which have values in itself? Can republic or empire issue as much of galactic credit as it need in the war?
The galactic credit standard was established on Sojourn. From its inception, the credit was backed by the immense wealth of the planet Muunilinst and the InterGalactic Banking Clan (IGBC). During the Clone Wars, the IGBC backed the currencies of both sides, as it would do again decades later for the New Republic and Imperial Remnant.
In the current canon, it seems Galactic Credits are government backed, with Republic, Confederacy, and Empire credits only being valid within their own system, although there was a degree of transferability when power shifted.
After the formation of the Galactic Empire, Galactic Republic credits were phased out within days, with Imperial citizens induced to register for identifying chain codes in order to exchange their now-worthless currency for Imperial credits. Eventually, credits from the time of the Republic became mostly worthless. Credits from the Galactic Empire likewise became worthless following the rise of the New Republic, to the point that Grand Admiral Rae Sloane was forced to pay the bounty hunter Mercurial Swift "in owed favors." The New Republic also issued credits, but their value became volatile in 34 ABY following the Hosnian Cataclysm. Consequently, the city of Canto Bight prohibited New Republic credits and required all visitors to exchange their credits for currency made of aurodium.