If it's at least bubbling some, it's probably "good enough."
Is this a partial or full boil? If you're trying to boil 6+ gallons, it's gonna take some jiggerpokery to get it up to a full, rolling boil on any home kitchen stove if you're lucky; the un-lucky never get there.
That being said, you can try boiling with the lid to your pot mostly on -- you don't want it all the way closed, because one of the things the boil does is drive off compounds that would otherwise produce off flavors (and will indeed do so if you boil with the lid entirely on, allowing everything you boil off to just condense on the lid and drip back down into the wort). But leaving it an inch or two cracked will allow most of the steam to escape while still keeping cool room-temperature air from circulating and leeching heat away from the surface of the wort.
Another thing you can try to do is position the pot over two burners, but depending on the size of your pot and the layout of your stove, you may or may not be able to actually get more burning gas under the pot this way.
I tried a couple of batches on my overpriced mid-2000's stove, then gave up and upgraded to a Blichmann burner hooked up to the natural gas line the previous occupant ran out to the deck for his gas "grill," and, as Robert Frost once said, that has made all the difference.
Edit: yeah, also, what they said about partial boils or splitting into a couple "sub-batches." If you can't get two or three gallons (per burner) to a rolling boil on a gas stove of any vintage, your beer is the least of your worries.