The G and F-Threads refer to the thread for nozzel guards/holders and gun extentions also know as the diffuser thread. The G- Thread is a larger 7/8" connection and the F-Thread is a smaller 11/16" connection. The two threads can be visually identified by looking at the distance of the thread to the end of the diffuser.
If there is a noticeable distance (roughly 5 mm or more) from the end of the thread to the edge of the diffuser then you have a G-Thread.
If the thread runs up to the end of the diffuser leaving no gap then you have an F-Thread gun. (Refer to image).
All Vector guns have a G-Thread.