Ranking metric in GSEA
If using DESeq2 with GSEA, I'd recommend ranking by shrunk log2FC values.

It'd also be worth considering ranking positive and negative associations separately, because the standard GSEA algorithm doesn't cross at the zero point when associations change from positive to negative.

You shouldn't be using p-values to rank anything. A p-value gives information about the reliability of a result, but not its importance. See point #5 of this article about p-values:

https://amstat.tandfonline.com/doi/full/10.1080/00031305.2016.1154108#_i31