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wiring of metal over poly gate

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Basically, yes, you can route metal layers across polygate, there is no DRC rule preventing this. In some cases, like where you have matched or noise sensitive circuits, you may need to consider where you route your paths in order to prevent parasitics.
And in addition to eklikerommys: some researchers/fabs have also demonstrated that the standard deviation of the mismatch between transistor pairs increases if you route metal on top of them. This was quite some years ago, and I do not know the relative numbers.
Wire can be routed over poly gate of digital circuit.
However, for sensitive analog circuit, specially for differential input pairs, it is inhibited.
There are plenty of techniques and plenty of formulas for it. And it is difficult to compile in an forum like this.

Quite a lot of work on this topic is actually found in publications on current-steering DACs. This type of DAC consists of a lot of unit current elements (i.e., transistors and bias trees). How to lay out and match 2^N different transistors/current sources and what impact it has on performance. There are some quite elegant formulas around.

Further on, check Pelgrom and Schouwenaars contributions to the field of matching of transistors.1
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