a table spoon of salt per gallon is still not enough... a table spoon per gallon would still not be enough

around 1/4 cup per gallon should get you to around 1.011sg. but each brand of salt will differ, as will the 1/4 cup. you still need to check with a hydrometer as the 1/4 cup will only get you in the ballpark.
you
need a hydrometer if your keeping fish that need salt. you wouldnt keep tropicals without a heater would you? if you have a hydrometer you know where you stand when it comes to salt content. having to guess the salt content is one more thing to cause grief to brackish aquarist's. its hard enough with the lack of reliable and conflicting info.
salt absorbs moisture from the air, so your table spoon of salt one week will be as is, but next week it could have absorbes x amount of water, throwing your levels off. it gets worse the longer you leave it. hydrometers measure the weight of the water + salt. a liter of fresh water weighs 1.000 Kg, where as Salt water is heavier. sea water is 1.022 Kg+ per liter.
you can raise your tank 0.002sg per week till you hit whatever level of salt you require. dont go any more than 0.002 per week though, as your cause your tank to crash. and puffers dont like that.
fluctuating salt levels is good practice. it mimics the natural environment of the GSP. the SG of brackish ecosystems fluctuates constantly. when the tide comes in, the hight of the tide, amount of freshwater coming down the river ect...
nice puffer BTW. he has a nice white stomach. the sign of a happy GSP. has a full stomach too. its deffo well fed