try looking for the gauge file on your computer cfg edit may have moved it to the folder it was loaded in.
A complete uninstall reinstall should have corrected this problem.You can fid out the name of the gauge by looking in the panel cfg file of any of your B737's.
If I am not mistaken it is what I call a master file NOT A .CAB file. One folder that has several gauges in it.
Like B737.gau for example.
You can also find all of the default gauges on disk 1 just unzip the GAMEONE.CAB file into a dummy folder of your choice. But Please DO NOT UNZIP INTO FS2004.
Once unzipped you can just drag all of the gauges from it to your gauge folder And over write. DO NOT MOVE THE GAUGE FOLDER just the gauges in the folder.
If you move the gauge folder and overwrite you will loose any added gauges you maight have put there.
