Welcome to FT and the BA Board

From Gold, you never go down to Blue - you always get a soft landing back down to Silver first for a year. Can you be more specific about the dates involved here between membership years? BA were being quite generous earlier this year renewing people's tiers due to the strike/disruptions/uncertainty at the start of the year meaning lots of premium pax switched business elsehwere to be more certain of their travel plans. It could be you got lucky in this sweep and therefore benefited.

The AmEx would not have anything to do with it.

This year (your current membership year), if you earn between 0 and 1495 (since, AFAIK, you can't get any closer to 1500 than that ) you should drop down to Silver. If you earn 1500 or above, you stay Gold. If you do drop down to Silver then in the next membership year, you need to earn 600 or more points to keep Silver, 1500 or more to go back up to Gold.