This is an image of one of the window trim. Should I paint it all? Gel stain it all?? Leave the windows and strip the stairs? I need help.

If you prefer the look of natural oak, then strip, sand, tack cloth and restain the stairway rails, etc... You may need to custom mix colors (pickled oak and golden pecan, or something similar) to attain the match, as the current stain may leave an orange undertone.

It’s a big undertaking either way, much of it on a ladder and getting in between spindles will be no fun, especially the sanding. If having done professionally, it won’t be cheap.

If budget would allow, I’d do something like below (minimal sanding/restaining) Given the staircase is a prominent feature, this would really update your space. Maybe getting a quote would help you decide.

If you strip them I'd stain to match the beam and fan, since eventually you may decide to paint all the oak trim.