When the Gold Bug Pro and G2 were introduced I was a Teknetics Dealer, used the Omega 8000, and preferred the round 8" Concentric and 5" DD on it. I got the G2 and, not caring for the 11" BiAxial (aka DD) I used a 5" DD and also mounted the 10" elliptical Concentric on it, and it did fair but not great.
The F-19 & G2+ were improved versions to replace the Gold Bug Pro and G2. I have owned, and parted with, four of the F-19's. They work better than th Gold Bug Pro, but I still preferred thw 5" DD on them. The only Concentric coil I tried on one of them was a round 7" Concentric Fisher coil. I had it on an F-44 I ordered, bought one for an Omega 8000, and had a 3rd 7" Concentric. I used in on an F-18 for some Coin Hunting in an urban environment and it worked. I liked it better than the 10" elliptical and the 11" BiAxial.
That said, I will tell you it worked for me, but it didn't work great, nor nearly as well as other makes and models I used with a 7" Concentric, such as my Omega 8000 V4, F-44, or Makro Racer II.
Monte