If you have a spare black output from the GV-100, run it to the Genlock input on the JVC camera. There should be timing adjustments (H-phase and SCh phase) in the camera menu that will allow you to adjust the camera so the video looks correct going through the switcher. Most Grass Valley 100 switchers have a blanking processor circuit that will smooth out the differences in timing. Not sure if there are any different issues for a switcher over in PAL-land.
Another not as correct but workable way to do it is to loop the video from the camera through the switcher input and then back out and into the REF input on the back of the switcher. Depending on the options on your switcher, there should be controls on the edge of one of the switcher cards (there should be three of them) for adjusting the timing of the switcher to the camera. A waveform monitor and a vector scope will make this much easier to do. If you have the switcher manual, look up the maintenance modes in the book. These will help in timing the signals through the switcher as well. You usually flip a toggle switch in the card edge and then push a reset button to get into the maintenance mode. The manual will have the rest.
Reading you post, it looks like you have the Grass 100 switcher and you are trying to feed it with the output from your camera and also the output from a DVD player. You are having difficulties getting the two sources timed so that you can do transitions between them on your switcher. Do I have it right so far?
I would take the spare black burst from the switcher chassis and run that to the sync input on your camera. Use the menu controls to time it into the switcher. Are you familiar with how ti time input sources to the switcher?
For the DVD player, the way you have it hooked up, the DVD player is essentially acting as your sync generator. Although it could work, it's not the best arrangement and also requires an optional external reference input card for the GVG 100 switcher in order for the switcher to "genlock" to an external reference. Does your switcher have this option? It's been a while since I worked with a GVG 100 switcher. I think it was called the sync regenerator option or something to that effect. It will allow the switcher to lock to an external reference source.
Probably the best way would be to get a frame synchronizer for your DVD player and lock the frame sync to the switcher reference the same as the camera. Frame syncs are fairly cheap these days and are readily available used on ebay. They can be pretty cheap since there are a lot of them being retired as facilities change over to digital or high definition.
A master sync generator or a good black burst reference generator would also be helpful and make the entire setup much more stable and easier to time. With this, you can send each device it's own reference signal and all of the parts of the system will be locked from the same reference signal.
The Leitch could work for you depending on what options the switcher has and what outputs the Leitch unit has. If the switcher has the sync pulse regenerator option, you can use a black output from the Leitch to sync you camera to the switcher. The DVD player will still require a Frame Sync if you need to mix or key over it through the switcher.
Explaining how to time an analog system through this forum would be difficult. You might try and find someone locally who is familiar with analog systems and tap them for the information. It's easy to show to someone but difficult to explain through emails.
Could you help me? I have a Grass Valley Video Switcher 100 also. The good news is I have all or most the components. Cameras, monitors, TBC, sync generator, etc... it all came out of a church production setup. Only problem for me is, they just took it apart and packed it. I don't have a clue how to set it back up. What goes where and in what order would really help me. even in a basic setup.