Hi,
I'm sorry I don't know anything about metformin. I'm currently diet controlled but have been threatened with insulin injections, no one has mentioned this drug to me.
Firstly I'd suggest seeing a phamacist tomorrow. They are highly qualified (four year full time degree) and understand the chemical formulation behind drugs, why and how they work. You should be able to find one on a Sunday whereas I guess you'll need to wait until next week for the GP.
Maybe you'd be better off on insulin given your (understandable) worries. In the meantime can you improve your diet control? I don't know how far along you are but I've only found the diet control difficult since 36 weeks, before that so long as I was really strict I was ok. I find two slices of toasted granary bread with two scrambled eggs is a low GI breakfast.
I did find the wiki page on it here
It says the following regarding GD:
Several observational studies and randomized controlled trials have found that metformin is as effective and safe as insulin for the management of gestational diabetes, and a small case-control study has suggested that the children of women given metformin instead of insulin may be healthier in the neonatal period. Nonetheless, several concerns have been raised regarding studies published thus far, and evidence on the long-term safety of metformin for both mother and child is still lacking.
From what it says about lactic acidosis I think it's unlikely because it is very rare and seems to be associated with metformin being used in combination with other drugs:
The most common adverse effect of metformin is gastrointestinal upset, including diarrhea, cramps, nausea, vomiting and increased flatulence; metformin is more commonly associated with gastrointestinal side-effects than most other anti-diabetic drugs. The most serious potential side-effect of metformin use is lactic acidosis; this complication is very rare, and the vast majority of these cases seem to be related to comorbid conditions such as impaired liver or kidney function, rather than to the metformin itself.
I hope that helps, good luck and do let us know how you get on.