from gekko import GEKKO

m = GEKKO()
x = m.Param(-1)
# use abs to define a new variable
y = m.abs(x)
# use abs in an equation 
z = m.Var()
m.Equation(z==m.abs(x)+1)

# solve
m.solve()

print('x: ' + str(x.value))
print('y: ' + str(y.value))
print('z: ' + str(z.value))