First off, download ESE. You can get it from Sonicology, under the Utilities section of the Downloads page. Second, open ESE and switch the language to English. Third, click Tools > CheckSum Check. Fourth, click "Load MD/32x ROM" and browse to where your ROM is located. Open it. The box at the bottom should say "Checksum Incorrect". Keep clicking "Fix CheckSum" until the button greys (cannot be clicked anymore) and the box says "Ok!". This may take two clicks of the button occasionally. The checksum is now fixed and the ROM is now playable. There IS an option in Gens to turn off checksum checking, but it's preferable to fix it in the ROM. This has been the Idiot's Guide to Checksums, and I've been Quick Man. Thank you and goodnight.
Even though I wish it wasn't neccessary at this point, I think this should be pinned. By the way, it's also possible to do with Genecyst by clicking on "Fix Checksum", then "Dump ROM", and then renaming the "rom" file that will appear in the Genecyst directory to something like "sonic.bin" or whatever you want.
