The sale is the latest move by GSK to fine-tune its consumer healthcare business and follows the divestment of other over-the-counter (OTC) brands in North America and Europe.
Net cash proceeds from the transaction are expected to be approximately 135 million pounds and will be returned to GSK shareholders during 2012.
Shares in Aspen closed 5.9 percent higher at 122.89 rand, a record close and the biggest daily percentage gain in nearly three years.
Aspen seem to have once again shown that the they are the M&A gurus with this deal, said Nino Frodema, a fund manager at Momentum Asset Management in Cape Town.
What we find positive about this deal is that Aspen seems to be beefing up its category presence in analgesics, which is a top-selling category.
The brands being sold generated sales of around 60 million pounds ($96.4 million) in 2011 in Australia, South Africa and Brazil. They include the painkiller Solpadeine and anti-acid drug Zantac.
Aspen, the biggest generic drug maker in the southern hemisphere, said it would pay for half of the price with cash and half with debt.
Taking on further borrowings to fund the deal was a concern, said Momentum's Frodema, especially since the company already has nearly 10 billion rand ($1.3 billion) worth of interest-bearing debt on its balance sheet.
Aspen CEO Stephen Saad, who co-founded the $6.4 billion company in 1997, said the deal would give its fledgling Latin American and southeast Asian business a needed boost.
For GSK, the net profit realised on the disposal in 2012 - after deducting transaction costs and a profit deferral of some 25 million pretax because Aspen is a GSK associate - is expected to be 105 million before tax, or 90 million after tax.
GSK added it continued to plan to sell its OTC weight-loss pill Alli. This process has been delayed due to supply problems at a Roche Holding AG
The Swiss company manufactures the active ingredient, orlistat, which is also the active substance in Roche's prescription-only drug Xenical. Alli is a low-dose version of Xenical.
GSK first announced in February 2011 that it planned to dispose of non-core brands sold primarily in North America and Europe and representing about 10 percent of its consumer health portfolio, in order to focus on priority brands and emerging markets.
($1 = 0.6226 British pounds)
($1 = 7.7976 South African rand)
(Editing by Kate Kelland and David Holmes)