No you can’t. Gearing is very different in classic for different classes and specs, it’s not just <later tier / more ilvl = better>. Many classes in sod still using some t1/t2 pieces or even dungeon items (such as HoJ) in their bis/prebis sets, on one side. On other side, you can simply get higher gs by switching to gear thats not good for your spec, or not good generally (crafted FR/NR, ony attune neck with broken GS scaling) but high ilvl, and losing dps. Gearscore wasn’t designed for classic. It was designed for WOTLK where in 90% situations you use only last tier gear
Even if you have 4 tier 2 pieces like paladin DPS, your GS will still be at least around 1000. But if your GS is 800, your gear is 100% guaranteed to suck. GS is a great tool to filter out brats trying to weasel their way into getting carried through content they are not supposed to be doing.
Doesn’t matter, still works well
Not 100% guaranteed, if u raided in t1/t2 and then left, but have good set bonuses and gear enchanted, your dps can be more then t3 alt carried in naxxramas without enchants. 800gs can be different gear, if its just crafted/boe trash then yes. but if its t1/t2 with good set bonuses you can do competatable dps with 1k geared people (but ofc it also depends of class because different classes have different set bonuses)
The difference between 800 and 1000 gs is not 20%. People like you who think MC gear is enough to do Naxx and be entitled to Naxx gear are precisely the reason why people resort to GS addons
Sure, it can be 100% difference, can be 50%, can be 20%, but also can be 0%, it depends of exact gear pieces
The bigger problem is that people think, that MC gear is not enough to do even BWL or ZG, demanding 900+gs for it that is more ilvl then loot drop from that raids. ABout naxx, especially if we are talking about low difficulty, the bigger question is amount of sanctified pieces then gs
mate. i see clowns asking for a 1k GS to enter KC.
Even if their character (be it alt or main) is equipped worse than they ask for.
That is stupid indeed. However it is a direct outcome of classic dungeon finder system (more specifically, the lack thereof). If you do not automate matchmaking and let people make their own groups and rules, you inevitably get stupid rules that are toxic and / or exploitative
People just need to have the self control not to join such groups.
Btw, just for interest, inspected 15 random people in SW with 1k+ gs, and 5 of them dont have ANY enchants (even cheap) on all of their gear. But ye GStards will still continue using GS 
Yes it is possible to have high GS and still underperform, for example if your lack enchants or consumables or world buffs or if your gear doesn’t have the stats your spec needs.
However it is just physically impossible to have 800 GS and be decently useful in Naxx HM3. The ability to filter out undergeared brats trying to get a free boost is already enough for the GS addon to be useful
Intellect provide mana pool unless u want to press 2x button and be oom 
The problem is whit GS is its a messure of your gear and not skill.
Dosent mater how good gear you got if you always fail the mecanics.
Often is people whit low GS will do as you say and if all got good GS you often say nothing and think every one knows all alredy.