It is not true that the phoneme g is not original in Czech.
Not even the grapheme g is not "original" in Czech. However, it was used for a different phoneme (https://en.wikipedia.org/wiki/Czech_orthography#Voicing_assimilation):
For historical reasons, the consonant [ɡ] is written k in Czech words like kde ('where', < Proto-Slavic *kъdě) or kdo ('who', < Proto-Slavic *kъto). This is because the letter g was historically used for the consonant [j]. The original Slavic phoneme /ɡ/ changed into /h/ in the Old-Czech period. Thus, /ɡ/ is not a separate phoneme (with a corresponding grapheme) in words of domestic origin; it occurs only in foreign words (e.g. graf, gram, etc.).
G is a normal original Czech phoneme (https://fonetika.ff.cuni.cz/en/czech-phonetics):
The Czech consonantal system has 26 phonemes, but in total at least 31 speech sounds are commonly found as their realizations. This is partly due to the fact that Czech has a very rich system of regressive voicing assimilation, which applies both within words and across the word boundary.
Czech speakers cannot pronounce two consecutive consonants which one is voiced and the other voiceless - one is always assimilated to the other. And here often k becomes [g].
Examples: kde [gde], kdo [gdo], leckdo [ledzgdo]; also listen to the audio example "5) alveolar affricates [t͡s d͡z] – cena, leckdo" on the same site.
More detailed description is also in N.S. Trubetzkoy: Studies in General Linguistics and Language Structure: https://books.google.cz/books?id=M5w94-Yx1gAC&pg=PA274&lpg=PA274&dq=czech+assimilation+of+voice+kde+gde&source=bl&ots=DjcZdP5PtQ&sig=ACfU3U3WRQ-Ud8U7ber_gC1idhERa9McwQ&hl=en&sa=X&ved=2ahUKEwiY6buLyKzgAhWCFiwKHXkjBiMQ6AEwAnoECAAQAQ#v=onepage&q=gde&f=false
To paraphrase your question, I have no clue why you feel like the alternate form of “g” should be “dz” :) "Mádzích* a tadzích sounds just... quite funny (in both the meanings of the word), but not Czech.