To convert a regularly spaced data set to a Surfer grid, use the Home | Grid Data | Grid Data command. Since your data is already regularly-spaced, set the Gridding Method to Nearest Neighbor and set the spacing of the output grid to match the spacing of the input data. Since the grid nodes will lie exactly on the data points, no interpolation will take place.
Alternatively, Surfer 13 (and higher) can read uniformly spaced data in matrix format directly as a grid file. If your data is in matrix format, follow the instructions in the article I Have Data In A Matrix Form. How Can I Use This To Create A Map In Surfer? to create a grid based map directly from matrix data.
Updated October, 2021