Not that this administration cares, but an executive order mandating registration for all firearms would be a direct violation of existing federal law prohibiting a registry.
When the government first mandates and then enforces a registry for all newspapers and magazines, I may possibly consider registering whatever guns that I may have at that time. For about a second, until the laughter starts...and when it stops, I'll resume reloading. IOW,
I WILL NOT COMPLY. EVER.
"They" can have my guns only when said guns are significantly warmer than my then-cooling body, and I'd caution them to be careful not to slip on all of the empty brass that'd be surrounding my body.
To paraphrase King Leonidas I, "COME AND TAKE THEM, KENYAN!"
I.e. we could ignore it.