Maple is cautious with fractional powers since a fractional power of a complex number is multivalued in general.  To reassure Maple of your intent, apply abs() to y(x), as in

de := diff(y(x),x) = 3*abs(y(x))^(2/3);

1. In mathematical formulas the parenteses (), brackets [], and braces {} tend to be used interchangeably, but in Maple they have quite distinct meanings.  Go through your worksheet and replace all [] and {} with ().

2. The Maple syntax for defining a function like f(x,y) = x^2 + y^2 is

f := (x,y) -> x^2 + y^2

You seem to use the notation

f(x,y) := x^2 + y^2

which is incorrect.  Go through your worksheet and fix those as well.

3. Your equations contain a parameter Gr sub zero, but you have given a numerical value to Gr without the sub zero.  Why?

> ListTools:-Rotate([a,b,c,d],1);
                          [b, c, d, a]

> ListTools:-Rotate([a,b,c,d],2);
                          [c, d, a, b]

> ListTools:-Rotate([a,b,c,d],-1);
                          [d, a, b, c]

with(plots):

odeplot(dsn, [t,u(t)], 0..tmax);

See ?odeplot for more examples.