The Gundabad history comes from
Of Dwarves and Men, section titled "Relations of the Longbeard Dwarves and Men".
This opens with a discussion of the awakening places of the Dwarves, and Tolkien just drops it in like a bombshell:
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...the second had been Mount Gundabad (in origin a Khuzdul name), which was therefore revered by the Dwarves, and its occupation in the Third Age by the Orks of Sauron was one of the chief reasons for their great hatred of the Orks.
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Gundabad was also a meeting place for Dwarf assemblies (same source):
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Though these four points were far sundered the Dwarves of different kindreds were in communication, and in the early ages often held assemblies of delegates at Mount Gundabad.
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Gundabad as an Orc stronghold seems to have only occurred during Sauron's invasion of Eriador in mid-SA:
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The Second Age had reached only the middle of its course (c. Second Age 1695) when he invaded Eriador and destroyed Eregion, a small realm established by the Eldar migrating from the ruin of Beleriand that had formed an alliance also with the Longbeards of Moria. This marked the end of the Alliance of the Longbeards with Men of the North. For though Moria remained impregnable for many centuries, the Orks reinforced and commanded by servants of Sauron invaded the mountains again. Gundabad was re-taken, the Ered Mithrin infested and the communication between Moria and the Iron Hills for a time cut off.
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(Use of the phrase "re-taken" is curious here but I don't believe it's significant; this was a late essay from a time when Tolkien himself was admitting that his memory was failing.)