Is it insecure using addition instead of multiplication in Elgamal encryption? I am wondering why most textbook only explain Elgamal encryption using multiplication operation, i.e. $c = m\cdot g^{ab} \pmod p$ instead of addition modulo $p$, i.e. $c = m + g^{ab} \pmod p$?

Is there any flaws or insecurity for the addition version of ElGamal encryption?
In the generic sense of an abstract group, this is a problem since addition may not be defined. However, when working modulo a prime $p$, addition is certainly defined. However, it is not secure. In order to see why, note that we must work in a prime subgroup of $p$ in order for ElGamal to be secure. Thus, we typically choose $p=2q+1$ where $q$ is also a prime, and then we work in the subgroup of order $q$ of quadratic residues. When you add, you may get out of the subgroup.

A description of an attack when addition is used A Simple Attack on ElGamal Public Key Encryption by Dan Boneh (the paper deals with something else, but also considers addition as motivation).