“If I Love A Boy” is a set of 5 short movies that don’t have any common thread to them that stars the same actors as Thank God It’s Friday. TGIF is a full on multi-part series that builds on several storylines that run through the series. Additionally If I Love a Boy are more short dramas where as TGIF is a rom-com. Unfortunately, If Love A Boy is not a prequel to TGIF, nor do the actors have the same roles between the series and the short movies. This is unfortunate because the If I Love A Boy short stories seem to be more teasers with open endings that seem to suggest a larger series/movie in the future. How true that is, is purely speculation. Watch each for the same actors.
Thank god It's Friday is set in the same school with the same music club as Love Sick. The connection is shown in episode one. Also, both are set up as BL. TGIF has more characters and a set up for multiple couples it sees. Similar vibe between the two of them so far as well. The relation is slight however so I'm sure you could get away with watching TGIF without watching Love Sick. However, I would recommend watching Love Sick first because it's really good and having seen it you can more fully appreciate the setting of TGIF.