Royal Mail is the UK's equivalent of the US Post Office - Gorilla's shipping uses Royal Airsure which is trackable international mail.

Once shipped, it'll take about 1 day to pass from Royal Mail to US Customs - at that point, it's up to the federal government to release your package to the US Post Office.
No one can say how long that will take other than the US Customs department - it varies depending on what office it goes through, the volume of mail, whether there are any security alerts, etc.

After that, it's handed off to the US Post Office & I believe they treat Royal Airsure like priority mail so it should take from 1-3 additional days depending on how far it has to go.

On average, it should take between 10 & 25 days total - but as I mentioned, I recently received something in just 6 days from the time the package shipped which was incredibly fast. The shipments are trackable, but they don't require a signature at delivery.

I see your order & I've emailed the department that handles order fulfillment & shipping to see if they can give your order a little special attention until it gets out the door.