Well lets see
Return has base 102 power at full happiness, x4 attack, x 1.5 STAB, x 1.5 normal gem, you get a base power of 918. How much damage your slaking will do will depend on the opponent of course, so here's a couple of calcs on some commonly used Pokemon, assuming you have an attack boosting nature.
252+ Atk Normal Gem Slaking Return vs. 232 HP / 0 Def Rotom-W: 303-357 (101.33 - 119.39%) -- guaranteed OHKO
252+ Atk Normal Gem Slaking Return vs. 252 HP / 252+ Def Eviolite Chansey: 424-501 (60.22 - 71.16%) -- guaranteed 2HKO
252+ Atk Normal Gem Slaking Return vs. 252 HP / 252+ Def Donphan: 204-240 (53.12 - 62.5%) -- guaranteed 2HKO
This is in Ou of course, and I'm not even including Pokemon that resist or are immune to Normal type attacks, such as Skarmory, Jellicent and Tyranitar. Here are calcs for Nu threats, Slaking's tier
252+ Atk Normal Gem Slaking Return vs. 248 HP / 8 Def Altaria: 348-409 (98.58 - 115.86%) -- 87.5% chance to OHKO
252+ Atk Normal Gem Slaking Return vs. 252 HP / 252+ Def Alomomola: 267-315 (50 - 58.98%) -- 74.22% chance to 2HKO
252+ Atk Normal Gem Slaking Return vs. 252 HP / 236+ Def Eviolite Shelgon: 156-184 (46.7 - 55.08%) -- 69.14% chance to 2HKO
As you see, although Slaking's return is quite strong, but not strong enough to outright obliterate everything in it's path, even in Nu. Most defensive walls will be able to stomach it. However, most offensive Pokemon who do not resist it will fall to it. You may also want to try a choice band, as it provides the same power boost as a gem to all of slaking's moves for the entirety of the battle at the cost of being able to switch moves.
Here you can calculate how much damage your Slaking will do on practically any Pokemon. Here is the complete damage formula for Black and White. Best of luck to you
