We have three options for a solution here:
1) the one you brought up, creating a girls'_frontline_(series)
tag which can encompass the original game, the spinoffs and the sequel; would probably be among the simpler solutions, as you can just plop it on all posts under Girls' Frontline at the moment and do some minimal gardening.
2) creating a girls'_frontline_(game)
tag and turning Girls' Frontline into a series tag by itself; would require far more gardening, being the most intense solution here, but it would keep the overall tag looking clean; or alternatively just alias the spinoffs and sequel and just not care for the consequences (especially as one could wonder to what extent some characters from NC or GFL2 would be popular enough to flood the tag).
3) repurposing the Reverse Collapse (series) tag, and turning it into a general series tag rather than one just used for in-series crossovers and the Codename: Bakery Girl remake; would be similar to 1), as it would necessitate adding it to all posts under Girls' Frontline, and still require the same amount of gardening as 1), but would theoretically avoid having to create a new tag (assuming a new tag isn't made to distinguish between the original Bakery Girl and its remake).
Of course, there are some side problems to consider as well, which could have different answers depending on the approach that we take; for instance, the issue of whether or not those characters appearing in NC from GFL should receive another chartag dedicated to NC, because they'll often have different attires/designs, and of course, the aforementioned potential flooding issue.