Where to start...
Heat is fine, and to be expected.
Range is probably fine, but it could be a little longer. You're just giving +1 S, +2 M, and +3 Long. That doesn't justify the mass when the HGR is the hardest hitting weapon for the same weight.
Mass-wise, most ER weapons don't increase in weight when going ER, however, Gauss weapons are a little bit different, and increasing Heat by 100% really doesn't amount to much over all. It's a good way to address it.
Crits is probably just 1 point too much unless you want to force no Hands while carrying this in the Arm. True, in most cases when a Gauss Rifle is in the Arm they usually leave the Hand off, but it isn't required with the standard Gauss Rifle. 8 would probably work here.
How much does it explode for when critically hit? It should be more than a Gauss Rifle, but not as much as the HGR. The increase in damage should coincide with the increase in range.
What does, "generates plasma at short range when fired", mean? Anything in Short Range gets D6 Heat or equivalent Damage? No other Gauss weapon does this, so unless you're literally adding Plasma foam Ammo to it, there is no reason to include this.
To-Hit modifier should not be there. All other To-Hit modified weapons are due to being mult-shot/multi-fired weapons (i.e. Pulse and Cluster). The "To-Hit Modifier" of ER weapons comes from putting more room where the different Range modifiers are engaged. If anything, we've seen To-Hit modifiers be worsened with ER weapons more than improved (see Clan ER Pulse weapons being -1 instead of -2).
