There is a sweet spot, when the GSP is good value ...... for most other cases it ain't. Buying multiple lots is one of the cases when it ain't. You always get charged postage costs on each item, so I would never recommend making multiple purchases off a Seller using the GSP. Maybe contact the Seller first to see if they would consider changing it or something, but they might not think the hassle is worth it.
Some people do get very upset about the GSP; I see it as a tool which is sometimes useful and sometimes not (mostly not for me in fairness). Essentially you are getting a tracked delivery system which is expensive, but probably in line with other international tracked delivery systems, which are also expensive. As a Seller the plus is that you don't have to worry about pricing up delivery costs to every single country on the globe and offering that as your international shipping costs, eBay automatically slaps the GSP cost onto your listing for you and you only need to send the parcel to eBay in your own country and they take it from there, including any additional fees. As long as the parcel gets safely to the GSP depot, you are fully insured against loss as a Seller (which is good peace of mind when shipping to countries with less reliable postal services). The down side is the multiple purchases as already mentioned, but also that GSP puts a lot of Buyers off because it's an expensive postage option - great if you're selling expensive stuff internationally (so Buyers expect hefty postage costs), not so good if you're selling stuff you expect to sell for a more modest amount with a modest postage cost.
Personally, I find international shipping costs tend to be around the same amount, give or take, so as a Seller I would quote a ballpark amount for shipping and charge that whether it's going to the US or Uganda.