take a coat, and a OS map other than that its not a difficult ride and the ground should be pretty dry as 90% of the tracks are armoured and drain well.
Only tricky point is at this bit, as I have ridden it a few times and still miss this once in a while
Follow this is the general direction of the disused Trefil Quarry that should be on the skyline in front of you. When you reach a hard track turn right and follow it around and up the hill. Continue along it until you see a faint grassy track veering off to the right hand side.
The trails are very faint and very very easy to miss, and if you over shoot them too much and try and cut across to them using your OS map you will come across some very deep gulleys with streams in them (not rideable).
So if the weather is shit, or its a bit foggy where you get to the 4×4 ruts, you can follow them across the moors to the edge of a forest, go through the gate and descend down to the outdoor pursuits center (nice rocky very fast descent here) then before you go under the old railway bridge take a right up a steep grassy path to a fireroad turn right follow this to the end (its a couple of miles) and you will hit a road go left then right take the next bridleway and this drops you straight onto the gap climb.
If you look on a map its all pretty obvious.
edit: also for some strange reason, this ride seems to cause the most mechanicals in the group/groups I have ridden in, so take some bits to do trailside bodges as its a fair walk out.