To merge two or more grid image files into one access the Grid Utilities plugin from the Gridding menu and choose the Merge option from the Utilities button.
If the grids have not already been loaded into the current session open them by selecting the Open button (yellow folder) at the top of the screen.
The grids will then also be added to the Grid: drop-down list. Select each grid individually from this list and click the ADD button after each selection. The grids will then be added to the "Grids to merge" section.
The method for how the overlapping grid cells will be handled during the merge process is set to Average by default. Choose between this method or using the Minimum, Maximum and Sum options listed.
Click the Save As... button in the bottom right hand corner to enter the name of the resultant merged grid.