sleepspaniel perfectly reasonable question :)
IF a baby is not growing well enough then this may be because s/he's not getting enough milk, or it may be that there's an underlying health problem.
If there is a health problem the GP is well placed to deal with this.
If the baby isn't getting enough milk, and it's a breastfed baby, the first answer is not to give formula, but to work on improving the breastfeeding. GPs don't get trained in breastfeeding in any way that is any use as part of their standard training, so they are unable to help with this (unless they have had separate training of course). What you ideally need is a breastfeeding-trained person, ie a breastfeeding counsellor trained with the NCT, ABM, LLL or BfN (3 year diploma or equivalent) or IBCLC.
Unfortunately, rather than recognising or accepting that they can't help with improving breastmilk transfer, GPs VERY OFTEN (of course not always) will simply suggest all they do understand - ie offer formula top ups.
I didn't mean the use of top up EBM - however while this can be a useful tool it's also not the ideal option in many cases as it's a PITA and often improving direct BFing milk transfer is a vastly superior solution. It is, of course, an option to be looked at in many cases though.
Of course, sometimes formula IS required. But it should be the last, not first option. Simply topping up with formula can seriously undermine breastfeeding, and has its own significant health risks (which have to be countered by the risk of not topping up where it is genuinely indicated).
So, to try to answer your question, you are absolutely right that the parent of a slow-growing baby would be well advised to see if they can get the baby to take more milk (which you'll notice I suggested in my original reply) but my personal gripe is with GPs who simply immediately suggest formula top ups.
Saying all that, rottenrow, I still see nothing in your OP at all that would cause me concern - however always be aware that here online we simply can't see the full picture so do not hesitate to seek RL medical advice if you feel at all concerned.