Yes, I'm just telling you how to make groovymame synchronize properly in that specific case (also add "autosync 0" to mame.ini, btw).
Didn't tell you how to make groovymame pick that video mode since I thought you already knew how to do that.
Your OS will be reporting your custom resolution by 3 integer values, e.g. 640x480@110. So you have to force that mode through the resolution option, e.g. -resolution 640x480@110
That assumes the option -switchres is enabled. But enabling switchres requires you to define a monitor preset that is compatible with the mode you expect to set. This is by design to block unwanted frequencies. So it's not that simple. I'd need more information about the resolution you created.
Since you just mentioned "my graphics card" without a label I'm assuming it's not a supported AMD card, correct? If this is the case, you could just run baseline mame without the switchres complication.