Use the inpolygon function to determine which points are inside the boundary you define, then replace those points which are outside with NaN.
[X, Y] = ndgrid([-180:0.25:179.75],[-90:0.25:89.75]);
Z = griddata(...); %on the same grid as X and Y
boundaries = readmatrix('boundary_file') % replace with actual boundary file
% may need to change file-reading function
boundary_x = boundaries(:,1);% x values of boundary file
boundary_y = boundaries(:,2);% y values of boundary file
in_bounds = inpolygon(X,Y,boundary_x,boundary_y);
X(~in_bounds) = NaN;
Y(~in_bounds) = NaN;
Z(~in_bounds) = NaN;
