Gorillas can get pregnant from about the age of 9 years. They're pregnant for about 8.5 months.
As long as the kid is still drinking milk, mom's menstruation is absent or suppressed. And even though kids start to eat regular food from when they're only a few months old and eats more and more when he/she gets older and older, a kid still drinks milk now and then till he/she's about 3 / 3.5 years. So, all that time mom can't get pregnant again (even though there are some exceptions, like gorilla Tamani in Rotterdam Zoo). When the kid is weaned, mom starts to ovulate again and often becomes pregnant soon.
As an example I take gorilla Mandji, born in the wild in 1975, living in Apenheul. Here you can see the dates of birth of her kids:
» 10 Dec 82
» 16 Oct 86
» 5 Oct 89
» 28 Apr 1993 (died 27-9-1993)
» 19 July 1994*
» 17 Apr 1998
» 23 Aug 2001
» 15 Oct 2007**
» 10 Jan 2011
*Since the baby born 28 Apr 1993 died, she already gave birth to another one a year later.
**The silverback she lived with all her live, Bongo, died on 14 Aug 2005. As a result of that there wasn't a silverback for a while. They had to search for a new one, not related to Bongo and any of the gorillas living in Apenheul. They finally found one in Krefeld, Jambo (at that time 13 years old). End 2006 Jambo moved to Apenheul. That's why there was a longer time between the 2001 and 2007 kids.