GMG has been around for a while. They bought playfire (social gaming network) a while back. They also publish games now. I think it's only been crappy indie games so far.
Gamesplanet is owned by Metaboli, which is a French company. They've been around for a while. The also own gametap. they rarely have a good sale like this. They have regional sites, which actually have different prices.
https://de.gamesplanet.com/ German
https://uk.gamesplanet.com/ UK
https://fr.gamesplanet.com/ French
for example on the german site doom 3 bfg edition was <$3 but on the UK site it was something like $6. it's kind of a hassle to convert the units to USD so if you want easy deals i'd stick to GMG. Everything on GMG is usually a rest of world (row) steam key so it'll work anywhere. Not 100% sure about Gamesplanet.
They will have an encore day though so if you're interested in the indiana jones point & click games I'd pick them up. Last time they were on sale for 50% off was like 2 years ago or more.