Hi Sanzhar,

Use Epilog rather than GridLines. e.g. with only the vertical lines

Plot[{Subscript[A, max], Subscript[A, n0], Subscript[A, n2], Subscript[A, n3]}, {p, 0, 550}, 
 PlotStyle -> {{Thick}, {Dashed}, {Dashed}, {Dashed}},
 AxesLabel -> {p[c^-1], A[м]},
 PlotRange -> {0, 0.007},
 PlotLegends -> 
  LineLegend[{Subscript[n, 1], Subscript[n, 0], Subscript[n, 2], Subscript[n, 3]}],
 Epilog -> {
   {Blue, Dashed, Line[{{pmax, 0}, {pmax, maxA}}], PointSize[Medium], 
    Point[{pmax, maxA}], 
    Text["Arez", {pmax, maxA} + {15, 0.05*maxA}], {Red, Dashed, 
     Line[{{k, 0}, {k, Subscript[A, rez]}}], PointSize[Medium], 
     Point[{k, Subscript[A, rez]}], 
     Text["Amah", {k, Subscript[A, rez]} + {-25, 0.05*maxA}]}}
   }]

The points of intersection are known, so just add those line segments to the Epilog e.g. to draw the horizontal red line.

{Red, Dashed, Line[{{k, 0}, {k, Subscript[A, rez]}}], 
 Line[{{0, Subscript[A, rez]}, {k, Subscript[A, rez]}}], 
 PointSize[Medium], Point[{k, Subscript[A, rez]}], 
 Text["Amah", {k, Subscript[A, rez]} + {-25, 0.05*maxA}]}

Here's a graph that's turned out. Thank you more for helping me.