The TYPO3 GMENU does usually use the title field for the altText, how to change that shows the following snipet.

In this examle the subtitle field is used. If the subtitle field ist empty the title field is used.

temp.menu=HMENU
temp.menu.1=GMENU
temp.menu.1{
  disableAltText=1
  NO {
    allStdWrap.insertData = 1
    imgParams= alt="{field:subtitle//field:title}"
    ATagTitle.field = subtitle//title
    backColor = #ffffff
    XY = [10.w]+15, 18
    ... - ERROR: Line 11: Object Name String, "..." was not preceded by any operator, =<>({;Line 27: Object Name String, "..." was not preceded by any operator, =<>({
  }            
}