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XOOPS does 'flush' the tables but unfortunately the module developer has to tell XOOPS which tables to 'flush'. Sounds like the module has a bug in it's xoops_version.php file. If you want to play with the RM+Gallery you could just edit the xoops_version.php file...
1) Find the area in xoops_version.php where the SQL tables are listed. You'll see entries like
$modversion['tables'][0] = "rmgal_{some value}";
2) Add another table (right after the last one listed, increment the number to be one more than the last 'digit' [index] listed)... so it's something like
$modversion['tables'][{n+1}]="rmgal_categos";
where n+1 is one larger than the last index in the list.
Sorry, I don't have a copy of rm+gallery installed or I'd tell you the exact location. If this isn't clear enough just past the $modversion['tables'] section from xoops_version.php here so we can tell you exactly what to put in the file...