1) The head does not grip the line (in most cases). The head rotates via pawls and gears. By the head rotating through the clicks on the gear, it is unwinding the line from the spool. The spool usually holds the line around a post or something else that grips the line at the halfway point. All the methods of gripping I have seen do not care what size line is used.

If you use the precut type of head, that may be a different story. I have never used that type.

2) 89 Octane or higher is recommended on the engines. Mixing every month is good but not always possible with HO's. If your gas will sit for awhile, use Stabil. It will keep the gas fresh longer. Also store your gas in a cool place, out of direct sunlight.

3) You can probably find an owners manual on the Echo site. About all I do for my Stihl is keep the gas from sitting too long in it, replace the string, keep the filter clean and grease the hub every year. No probs so far @3-4 years of light to medium use.

GT2000R came from Echo with .080 line. I think you'd be OK with .095 if you want something heavier, but would not recommend .105. The engine is 22cc and .105 is a little too heavy.

Since the GT2000R came with pre-cut style head, should grip .080 and .095 OK.

Personally, I prefer a bulk-type, manual feed head. I have an SRM2000 (same engine as yours, but straight shaft) and use .095 very successfully. I only have to advance the head two or three times when edging and trimming, and since it holds 20 to 30 feet, I only have to reload once a month or so.