Dwarfs are intelligent creatures, with stronger bones and muscles but smaller than humans. They live mostly under ground (caves, mines, etc.) and reproduce sexually. How long the gestation period would be for a female dwarf? Dwarfs were hunted by creatures that fed mainly on them and had to adapt to overcome that (animals at the bottom of food chain usually have a small gestation period)
If you are assuming that your dwarves are still primates, you can make use of the following chart, relating the gestation time (in days) with the weight (of a fully grown female in kilograms) for primates (source)
Which can be fitted with the relationship
$\log(Gest) = 2.075 + 0.189 (\log(Wt)) $
Estimate the weight of your dwarves, and get the fitting gestation length.
Gestation time in great apes varies from 240 days to 280 days on average. Not a huge range given hte size difference between the largest and smallest members, bonobos and gorilla. So anywhere in that range is fine. The longest gestation time is not gorilla but humans (the range is 240-260 days without humans) so you may want to favor the high end of the range, something about large brains favors longer gestation.