Hard Gel forms a very tight polymer network or 'mesh', so there is very little gap for water and/ or oils to penetrate. This is why you can not soak them off (the acetone will not penetrate the 'mesh) It will eventually work, but takes HOURS!
Cuticle oils obviously will suffer the same issue - They will help, but at a greatly reduced level - Also any with heavier oils will have even less effect...
However - Cuticle oils will still greatly help the skin condition and with the treatment of any exposed nail, and if enough is regularly applied it will have some minimal benefit to a Gel nail...
Obviously a gel top coat will form this tight polymer network over the top of the L&P - However I personally will always still recommend a cuticle oil even with a gel top coat
I hope that helps