A general linear model doesn't generalize the function of $X$.
Indeed assuming you mean $E(Y|X)$ where you have $Y$ (and independent errors) whether or not there's a transformed predictor doesn't change things -- either way it would still be called a linear model (the conditional mean is a linear function of the parameters).
That is to say, consider $\alpha+\beta \psi(X)$. Now let $X^* = \psi(X)$. Then in terms of this new variable (the one used in the estimation) we have $\alpha+\beta X^*$. So either a linear model or a general linear model will be able to incorporate a transformation, $\psi$, (of the independent variable or variables) without difficulty.
Instead, with a multivariate response (each observation point is a vector of values), a general linear model generalizes the covariance structure of the error term so that the response values includes the possibility of correlated errors within the observation vector (that is, the components of $\underline{y}_i$ are correlated).
[Between observations there is still independence; if you want instead to generalize to correlated errors between-observations, that would be generalized least squares as fcop pointed out in comments.]