I know them both well, although I haven't ridden either for over quarter of a century. A few years ago I was driving down the Tumble, probably after visiting the Big Pit with my family, and I astonished my kids by saying I was once able to cycle up this. At the time I was sure my cycling days were over and I couldn't believe it either.
Apart from the well-known B-road from Abergavenny to Blaenavon, there are two other minor road routes up the Tumble. I don't know the route from the east, but from the map it looks the most challenging option. The westerly approach, beginning at Govilon has spectacular views up Cwm Clydach which you won't see any other way, and for a while runs along the edge of an unfenced vertical precipice. So well worth checking out if that's your thing. I may be heading out that way later this week (not on my bike) and I'll try to get some photographic evidence.
I've only ridden the Gospel from the south. Most say it's an easy climb. Certainly it climbs gently for mile after mile, but don't be deceived; just when you think it's going to be a doddle all the way there's a sharp kick up to what you hope is the summit. It isn't, but a least when you get there you can see what's left.
If you get the weather, it's fantastic:
There a two other routes across the Black Mountains to Hay roughly parallel to the Gospel. Neither is quite as high, but they're great too, and I'd expect them to be quieter. From what the OP said, he might be experiencing at least one of those as well.
Edit: I've straightened out my easts and wests. Too cold for my brain this morning.
