We simply never get any information throughout this TV series about family relations between the gummi bears of Gummi Glen or facts about other relatives. But some breadcrumbs can be combined concerning time line:
Nearly all gummi bears were displaced across the sea by the humans beeing jealous of their advanced knowledge, and only a few remained to call the others when it is safe to come home again (mentioned in "Light makes right"). These gummi bears hide from people, therefore no human beeing have seen a gummi bear for about 500 years (mentioned in "The magnificent seven gummies"). That this period can be devided by 100 is confirmed by the appearance of Gummadoon, a great gummi city hidden by magic which only appears every 100 years for one single day (mentioned in "The knights of Gummadoon").
Over the centuries the number of remaining gummies decreased and most of their former settlements are nowadays abondoned (mentioned in "Up, up, and away" and "Day of the Beevilweevils"). And the six protagonists of this series are the last descendants of the gummis of Gummi Glen.
But we never get to know, how old they are or even what is a normal life expectancy of a gummi bear! Therefore we can not estimate, how many generations of gummis have lived in Gummi Glen since the gummi's exodus.
The sequence of the gummis concerning age is easy to guess for the younger ones but tricky for the adults. Zummi and Grammi were favorite schoolmates and therefore may be nearly of the same age (mentioned in ""Little Bears Lost"). When Grammi went to school, Gruffi was a baby and Tummi was not even born (mentioned in "Gummies Just Want To Have Fun").
German viewers may think that Grammi and Gruffi are a couple, but this misbelief is based on a mistake in the German synchronization: One time Grammi quarreled with Gruffi and said to him angrily "I'm goin'" to point out, that she is not of his opinion and will go with the other gummis (in "Zummi makes it hot"). But in the German version, this very sentence is translated with "Ich verlasse Dich", which means "I leave you!".