It sounds normal. Breastmiks comes out quite forcefully during letdown- much faster than a bottle of milk. A bottle of milk has the same speed all the way through so I wouldn't compare. Breastfed babies can be slower to gain weight if the charts they use are for formula fed babies. Topping-up with formula will make you produce less milk in the long run.
My baby is now 5 months old and does gulp for around 30 seconds but then settles down for the rest of the feed. This has been the same since week one. I remember when she was about 4 weeks old the shrewdest breastfeeding woman was counting how many sucks to swallows and it was 3 sucks to one swallow - she seemed to think it should be less but it has never caused us any problems
I get multiple letdowns over the feed - sometimes sounds like 1 gulp per suck (lots of milk), then slows down, then another letdown back to big gulps. DD is now 6 months and I remember that when he was small (1-3 months) he would take up to an hour to feed, as he waited for further letdowns. Don't know if this is normal experience at all - he was certainly a bit of a slow feeder at this point, and he's now much quicker.
Hard to tell when she's swallowing after those first gulps but I think it might be groups of 4 sucks.
Still we're trialing gaviscon this week to see if reducing her reflux helps with weight gain. As it doesn't cause her much distress we hadn't done much about it but have heard it can help weight to knock it out so...