How can I plot grid lines on heat map?

I've been working on identifying the cutoff points for the heat map generated with the elastic modulus and hardness plotted on the X, Y coordinates respectively. I'm currently working on using grid lines to separate the layers of interest in the heat map for further analysis and discussion but couldn't find resource to help get started. Can I get explanation for what the concept of grid line is and how it can be implemented to separate the layers of interest in heat maps? Here's the code that I have so far to generate the heat map:

N = 50;    %adjust as desired. This is the resolution.

Specimen_84 = readtable ('Specimen_84.xlsx');
X = Specimen_84{:,17};
Y = Specimen_84{:,18};
Hardness = Specimen_84{:,11};
Elastic_Modulus = Specimen_84{:,8};

tbl = table (X, Y, Hardness, Elastic_Modulus);

figure
h = heatmap(tbl, 'X', 'Y', 'Colorvariable', 'Elastic_Modulus');

figure
h = heatmap(tbl, 'X', 'Y', 'Colorvariable', 'Hardness');

[KSSV]

grid on

[Shao Yang Zhang]

I've been getting this error:

Error in Specimen_84_HeatMap (line 12)

h = heatmap(tbl, 'X', 'Y', 'Colorvariable', 'Elastic_Modulus','GridLines',':');