You can define a function in _helpers.tpl, something like below

{{- define "superchart.getPath" -}}
{{- if .Values.gateway -}}
{{- if .Values.gateway.contextpath -}}
{{- printf "/%s/%s/" .Release.Name .Values.gateway.contextpath -}}
{{- else -}}
{{- printf "/%s/" .Release.Name -}}
{{- end -}}
{{- else -}}
{{- printf "/%s/" .Release.Name -}}
{{- end -}}
{{- end -}}

And use it in your deployment.yaml (or any other file)

prefix: {{ include "superchart.getPath" . }}

The Go text/template language has {{ if }}...{{ end }} conditionals and you can put almost arbitrary content inside of those. The direct approach to what you've shown could be

prefix: /{{ .Release.Name -}}
  {{- if .Values.gateway.contextpath -}}
    /{{ .Values.gateway.contextpath -}}
  {{- end -}}
  /

Wherever there's a - inside the curly braces, it causes the template engine to consume all of the whitespace adjacent to it, so this should appear in a single string (even though it's written over five lines).

(Remember that you can use the helm template command to render a chart without actually submitting it to Kubernetes, which is helpful for debugging.)

The aesthetics of the template file can be tricky; there are other ways you can rearrange this to possibly be more readable. Another alternative using a temporary variable:

{{- $p := .Values.gateway.contextpath }}
prefix: /{{ .Release.Name }}{{ if $p }}/{{ $p }}{{ end }}/

@edbighead's answer breaking this out into a completely separate template is a good approach too: the /path/part URL layout is fairly visible there, compared to my suggestions here.