Hi guest!

D4240 = gingival flap procedure, including root planing - four or more contiguous teeth or tooth bounded spaces per quadrant

The code listed above does not have direct crosscode we are aware of, so you can either bill the "D" code on the medical claim (many insurers these days will process "D" codes when they are medically necessary services), or you can try the CPT code below and include a narrative report describing the procedure:
41899 - Unlisted procedure, dentoalveolar structures

As for the diagnosis code, that will depend on the reason the gingival flap procedure is being performed on that patient! For example, here's from ICD-10 diagnosis coding options:

K05.00 Acute gingivitis, plaque induced
K05.01 Acute gingivitis, non-plaque induced
K05.10 Chronic gingivitis, plaque induced
K05.11 Chronic gingivitis, non-plaque induced
K04.01 Reversible pulpitis
K04.02 Irreversible pulpitis
K05.211 Aggressive periodontitis, localized, slight
K05.212 Aggressive periodontitis, localized, moderate
K05.213 Aggressive periodontitis, localized, severe
K05.219 Aggressive periodontitis, localized, unspecified severity
K05.221 Aggressive periodontitis, generalized, slight
K05.222 Aggressive periodontitis, generalized, moderate
K05.223 Aggressive periodontitis, generalized, severe
K05.229 Aggressive periodontitis, generalized, unspecified severity
K05.311 Chronic periodontitis, localized, slight
K05.312 Chronic periodontitis, localized, moderate
K05.313 Chronic periodontitis, localized, severe
K05.319 Chronic periodontitis, localized, unspecified severity
K05.321 Chronic periodontitis, generalized, slight
K05.322 Chronic periodontitis, generalized, moderate
K05.323 Chronic periodontitis, generalized, severe
K05.329 Chronic periodontitis, generalized, unspecified severity
K06.0 Gingival recession

Hope this helps!