Look through the main site: http://tasvideos.org/EmulatorResources.html
The general rule is that you need to load the ROM that the GMV was based on (we will not
distribute or point to ROMs), and then you need to "Play Movie" or some similar command, and if the checksums match, the emulator will start playing the game just as the TASer did.
If the checksums don't
match, you probably used a slightly different version of the ROM: Occasionally game manufacturers would need to fix some serious bug in their games or (less commonly) change something found offensive (like the vaguely-Islamic chanting in the OoT Fire Temple and the crescent on the Mirror Shield), so they would recall whatever the stores still had in stock and silently release new versions of the cartridges; the scene sometimes tags these versions as "v1.0" or "v1.1" (I think sometimes the actual manufacturers did this too).
Another possibility is that your download was corrupted or that you did not get a verified good dump (marked by "[!]") of a game that had one.
Now if the ROM is the best known dump of the correct version of the game, you may still have a problem if the GMV was made for an old version of Gens and subtly relied on the timing of that specific version; Gens is notorious for this, as is DeSmuME.
Basically, rendering a game written for one platform on another with frame-perfect accuracy would take insane amounts of RAM or CPU speed, so emulator programmers, seeing that normal gameplay isn't interrupted by taking certain shortcuts in interpreting the code, decide to do just that, sacrificing accuracy for speed; I think I remember reading a post by the author of bsnes, the notoriously slow and memory-hungry SNES emulator (which is now the basis for our lsnes because of its extreme accuracy), that doing an absolutely perfect job of emulating the SNES would take 3GB of RAM and a fast CPU.
Anyway, as emulators are developed, sometimes the core engines are modified, usually in some way that improves the accuracy-efficiency tradeoff, and this usually causes those frame-perfect TAS input files made for older versions of the emulators to desync with the new ones (after a point, the game's state isn't where it was expected, so the main character appears to wander around aimlessly or meander through the menu or something); for this reason, and because it generally takes so long just to make an input file, when WIPs are posted or finished runs are submitted, often a note is made of the version of the emulator used and which versions synched.
Keep in mind, making a TAS that relies on an emulator glitch, especially one fixed in the latest version of the emulator, is frowned upon; on the other hand, you don't need to discard all your work and start all over again just because yet another new build was released (but you can try hex-editing, if you're meticulous about its effects).
The smaller text above was a bit less important for your immediate situation; one final note is that you can always try to add to the GMV file by continuing your recording, at least if it's a WIP, and the link atop this post will point you there too.