Yes you can continue using Global Entry even with pending I-751. I travelled several times.
The worst case you will have to see a human if machine tells you to.
Once I had full face recognition kiosk which never asked me to scan documents - it was seamless.
Other times is prompted me to scan document. I've read before LPR is supposed to use GC to enter the US. So with expired GC and extension letter it obviously didn't work and I had to see a person every time.
Now I learned you can use passport instead. I cannot test it now since I have a 10 year GC already.
I heard people updating their Global Entry profiles by extending validity if GC based on extension letter. I have not tried that since I was worried it wasn't right to do. Whenever I spoke to Global Entry personnel at enrollment center (I tried walking in twice), they said they cannot update system from their end with extension letter.
I'd try updating expiration in profile and entering on passport if you're brave enough. Otherwise you can always use GE line and see a person.
