grammar - Incorrect gerund to simple past conversion?

I'm a non-native speaker of English, so this might or might not be something very basic. Nonetheless, it's baffling me and I'd love some help.

A friend of mine wrote this sentence in a story for which I'm a pre-reader:

They crossed a highway along the river shore and then a bridge, leading them to a dirt road winding down a thin peninsula that jutted into the river.

In my mind, based on how my native Portuguese phrases things, I felt that "leading" gerund there could be replaced by the determiner plus simple past "what led", and the phrase would "sound" better:

They crossed a highway along the river shore and then a bridge, what led them to a dirt road winding down a thin peninsula that jutted into the river.

The "what" in there is supposedly a determiner referring to and meaning the whole of "They crossed a highway along the river shore and then a bridge".

However he told me, and I quote him, "your suggestion in this case would cause the sentence to make absolutely no grammatical sense whatsoever."

I'd like to understand why that's the case, what the exact rules are, and whether there's a correct way to do a gerund to simple past conversion in this and similar cases.

Thank you very much!

Answer

As a general point, in English we would say 'which led' or 'that led', but not 'what led'.

Both sentences make grammatical sense (if you replace 'what' with 'which'). The problem lies with uncertainty about what is doing the leading.

They crossed a highway along the river shore and then a bridge, which
led them to a dirt road winding down a thin peninsula that jutted into
the river.

This implies that the bridge led them directly to a dirt road.

They crossed a highway along the river shore and then a bridge,
leading them to a dirt road winding down a thin peninsula that jutted
into the river.

This implies that their whole journey, crossing the highway and a bridge, ultimately led to a dirt road.

Grammatically it's OK. The problem I have with it is actually visualising it. 'Crossed a highway' means goes from one side of it to the other. If you cross a highway along the river shore you are either crossing from land down to the river, or coming from the river and crossing onto land. Then you cross a bridge, which is presumably over the river but necessarily so. It would make more sense to say they went along a highway by the river shore and crossed a bridge which led to a dirt road.