Dear MHS,
1. we generally recommend to first look at some of the 2D examples included in ATENA installation before you start to create your first own one. The example 2D models are typically installed in

%Public%\Documents\ATENA Examples\Science\GiD\Tutorial.Static2D

2. Please understand all entities have to be in the X-Y plane - otherwise, GiD can not detect it is a 2D model.

3. Just guessing: you have tried to generate the mesh for an unfinished model, then continued your modelling.
Some lines not belonging to any surface at the time of your first meshing attempt and have been automatically identified as reinforcement at that time, are now edges of surfaces.

In that case, I recommend to check by displaying the reported conditions, and unassign the Reinforcement Elements/Nodes Identification conditions from lines that are not reinforcement bars.

4. If you still have problems, you can follow ATENA Troubleshooting, 2.1.1 to send us your model etc.

Regards.

Dear MHS,
by "linear 2D elements" you mean a 1D (bar) element, or linear mesh (=not quadratic), or the option to consider the deformed shape from the previous step (=not updating it in each iteration), see also ATENA Troubleshooting, 2.1.9 Does the non linear analysis of ATENA consider the geometrical nonlinearity (second order effects, like buckling or necking), or something else?

Point A is the point at the global coordinates entered under Point A x, y, z. Similarly, the position of Point B is defined by Point B x, y, z.

Please understand only a linear gradient can be generated / defined this way. A linear gradient is uniquely defined by the values at 2 points - which is exactly what you need to enter. Alternatively, you can define directly, without the generator tool, by setting the constant part (=value at the coordinate system origin) and the coefficients determining its change along each coordinate direction.

Regards.