Patrick,
I think you have a legitimate quandary. Your Gehrig deviates the most from authentic Gehrigs in a few areas: 1. the yellowish tint on the front and back of the card does not look like typical toning of an authentic '25 Gehrig. Here's an example:
https://sports.ha.com/itm/baseball/1...umbnail-071515. 2. The roughly even corner wear bothers me. It may have been fabricated to look aged. 3. The stock showing signs of wear are not typical of a 100 year old card.
It's difficult to tell from your pics, but based on these deviations from norm I think the odds of this being authentic are 10-15%.
I suggest you do a couple things. 1. Buy a black light (they're available for under $10) and see how it looks in a completely dark room using that device. If it does not fluoresce or only shows dim brightening under a black light here's a very good chance the paper stock is pre-1950s. If it fluoresces brightly it's a modern reprint. Though not every card that does not fluoresce is vintage (read this article:
https://sabrbaseballcards.blog/2017/...uthentication/), most are. Every card that does fluoresce is modern (post late 1940's optical brighteners were added to most paper and these brighteners will glow when exposed to UV light) as far as I know. You can compare how it looks under a black light versus other 20's or 30's cards or post 1950's cards if you have those in your collection. Depending on how this goes you want to go to step 2. Call SGC and ask them if they will give you a discounted rate to review it. It might not be worth it if it costs you over $300 to review it and in all likelihood it comes back as not authentic after review, so you're going to have make a determination based on an expected value equation. You could also sell it raw -- hopefully with all the appropriate caveats for transparency -- and let someone else decide what to do with it.