I tried to dial *143# using my #GlobeBlackBerryZ10withLTE and works just fine.
Did you check if the GCash App is available for Lumia 800 OS (is it Windows 7?)
I am afraid it's only available for IOS, Android and BlackBerryOS Devices.
You mentioned this concern has been a month already. I salute you for your patience, kasi ang pasensya ko sa Globe konting konti na lang.
Anyway, please try to visit any Globe Store, and tell them with all your might about your concern. And insist that you want an ultimate fix not in the next week, in the next month, in the next year, but in the next 24 hours.
Keep Calm. Keep WILL.
I am using Lumia 510, with WP7.8 OS. I have the same problem with you, *143# menu is not accessible with my device. I wonder why they NEGLECTED WP devices tsk tsk tsk.