GNAT always uses code expansion for generic instantiation. This means that each time an instantiation occurs, a complete copy of the original code is made, with appropriate substitutions of formals by actuals.
It is not possible to refer to the original generic entities in GDB, but it is always possible to debug a particular instance of a generic, by using the appropriate expanded names. For example, if we have
procedure g is
generic package k is
procedure kp (v1 : in out integer);
end k;
package body k is
procedure kp (v1 : in out integer) is
begin
v1 := v1 + 1;
end kp;
end k;
package k1 is new k;
package k2 is new k;
var : integer := 1;
begin
k1.kp (var);
k2.kp (var);
k1.kp (var);
k2.kp (var);
end;
Then to break on a call to procedure kp in the k2 instance, simply use the command:
(gdb) break g.k2.kp
When the breakpoint occurs, you can step through the code of the instance in the normal manner and examine the values of local variables, as for other units.