Gnomish Plots

As the PCs move through the mines battling the orcs, the gnomes are shuttled away to a "safe" area where they can be held. When the PCs take out the orcs on patrol, they can make stealth checks and perception checks to sneak up on the orcs in their safe area.
The gnomes will be waiting for some approach, so they can make perception checks to notice the PCs approach with advantage. If they detect their approach, they will seek to distract the orcs guarding them. They can play at some illness or accident in an attempt to bring them away from the entrance, or alert them to "a noise down yonder hall..." away from the party's approach. They can roll deception checks for success.
They will work at leading the orcs deeper into the mines, trying to spread them out thinner, and lead them further into the depths with promises of gold (deception or persuasion checks). The truth is that most of the tunnels in the mine, particularly the lower two levels, have not been mapped out fully since the gnomes purchased it. They will have to bluff if they get too specific in detail. On the other hand, since the mine has sat abandoned for over ten years, there is no telling what condition the mines or supports are in. And who knows what has crawled in to call this home...
Understanding all of this, the gnomes will try to keep leading their orc captors deeper into the mine, looking for a situation they can use to their advantage to escape. They will dangle the promise of gold and riches in front of the orcs to draw them deeper or to send them down any shaky passages.
This can be strung out as long as the DM wants to milk the possibilities. Unfortunately, short of doling out the gnomes as PCs, there won't be anyone actually playing them, but they might be able to drop something that clues the party into their plan, allowing them to shadow behind and pick their opening.